Rainwater. Let me begin with rainwater. Thirty years ago, clean rainwater was taken for granted as everyone's right, and fishkeepers spread plastic tarps to catch it. Now a European author tells you "rainwater is no longer suitable for aquarium use because of heavy pollution in recent years" (Ines Scheurmann, The New Aquarium Handbook, Barron's 1986, p21) and the Baensch Atlas barks: "Never use rainwater." Heavy industries complain that restoring our unpolluted rainwater would unfairly eat into profits.

People avoid rainwater both for its acids (mainly sulfuric and nitric) and its pollutants. If you're considering using rainwater in your aquarium, you need to know just what the pollutants in your rainfall actually are. Perhaps their effects in the aquarium aren't so dire after all. Not all the pollutants in the atmosphere are even soluble, for a start.

The six most important categories of atmospheric pollutants being tracked by the Environmental Protection Agency are these: volatile organic compounds, carbon monoxide (CO), nitrogen oxides (NO and NO₂), which produce nitric acid and ozone, sulfur dioxide, lead and particulate matter, a.k.a. "soot"--— which is largely inert black carbon.

If sulfur dioxide and the nitrogen oxides haven't already reacted with water vapor in the air to form acids, they quickly break down in water, adding minute quantities of sulfuric and nitric acids, which are immediately neutralized by the carbonate buffering of your water and produce some sulfate and nitrate. Not really an aquarium problem, troublesome as they may be in the environment as a whole. Ozone also quickly reacts, giving its extra oxygen atom to the first organic molecule it encounters; when dissolved in water, ozone acts very much like hydrogen peroxide. So of the EPA's six trackable pollutants, the more dangerous components of rainwater would be heavy metals and the volatile organic compounds, such as molecules of unburned fuel, benzene, and the like, mostly recalcitrant chemicals that can't be scavenged by common bacteria. These have to be filtered out with activated carbon.

Recently (17 April 2002, New York Times), some fishkeepers were alarmed to hear from the National Academy of Science that traces of atrazine, Syngenta's common weedkiller used by agribusiness, are now found, not just in runoff, groundwater, reservoirs and aquifers, but even in rainwater. There is virtually no atrazine-free environment, the N.A.S. tells us. Though EPA rules permit three parts per billion in our drinking water, land-dwellers like us are well insulated from environmental toxins, compared, say, to frogs. Frog tadpoles are affected by atrazine at very low levels (0.1 ppb). Atrazine causes frog cells to produce an enzyme that converts testosterone to estrogen: exposed tadpoles developed multiple sex organs or were hermaphroditic, and testosterone levels in adult male frogs were decreased to levels more characteristic of females. Adult females weren't affected. Low exposure to atrazine also changed the rate of male to female daphnia. Later in the year, the same Berkeley team studied atrazine in wild frogs from heavily-exposed sites in the Midwest and the West, and found atrazine levels that fluctuate seasonally, 0.7 to 15.2 ppb.

The sex of mature fish, like that of frogs, is not sex-linked: fish have no male Y chromosome. Their gender isn't determined at fertilization of the egg, but is influenced by many environmental factors, not excluding atrazine. Atrazine is another occasion- not for anxiety- but for maintaining some fresh activated carbon in your filter-- and for filtering your rainwater too.

Filtering out particulates in polyester batting is easy enough. But because of the volatile organics that are likely to be in rainwater, I'd pass rainwater slowly through fresh activated carbon or better yet, PolyFilter. Heavy metals should be chelated with a water conditioner, such a "blackwater extract," or by peat filtration-- or a cup of green tea.

Many state public utility commissions are monitoring some further atmospheric pollutants: carbon dioxide (CO₂), methane (CH₄) and nitrous oxide (N₂O). These are greenhouse gases, important enough in the broader scheme of things, but they won't affect the aquarium quality of your rainwater. Methane isn't even very soluble.

Acid rain. Rainwater acids are quickly neutralized by your carbonate buffering. But just how acid is your own rain? In the New York area, rain during June-July 2001 averaged about pH 4.1, more than moderately acidic. I found these figures by going to the site maintained by the U.S. Geological Survey, which brings together on-line data and reports on acid rain and precipitation chemistry, notably recent levels of sulfate (currently lower than in 1980), nitrate (holding steady), ammonia (rising at a number of sites) as well as calcium, magnesium, phosphate, and sodium and chloride in precipitation. It's all gorgeously mapped for you, but there's little information about volatile organics in rainwater there, and the volatile organics are the most worrisome rainwater pollutants for aquaria.

Collecting, filtering and storing rainwater. If I'd recently been re-roofed, I'd wait a few months to allow any volatiles in the roofing materials to weather out, before I used rainwater. I wouldn't use rainwater that was running off a rusty old galvanized tin roof, come to think of it. And I wouldn't use rainwater that had lain in contact with a flat roof. But some other concerns, when rainwater is used as drinking water, don't apply. Certain organic roofing materials such as wooden shakes, porous tiles or concrete support cyanobacteria, algae and mosses that are harmless in the aquarium.

Frankly, if I lived outside center city (I'm in Manhattan), and as long as I wasn't in a major industrial pollution belt, and if my roof were not flat as mine is, but sloped, so that rain didn't lie in contact with it--— I'd be using rainwater to dilute the total dissolved solids in my aquarium water.

For collecting it, I'd have a snug-lidded modern plastic rainbarrel from Home Depot, which could be topped up, once the initial downpour had cleaned the air, using a swivelling end-section fitted to a gutter downspout. The part about waiting for the initial downpour, then running out in a rainstorm to shift the downspout is a definite drawback for some aquarists. Fancy modern rainbarrels have a trick attachment that lets the first cloudburst clean the air and the roofing and drain away, before it starts filling the barrel. I might not worry about these refinements and just let particulates settle out. Of course, I'd filter out soot and other particulates by passing the rainwater once through filter floss, and I would always filter it slowly through fresh activated carbon. I might also cycle it through a funnel filled with peat moss to bind heavy metal impurities. If I were still anxious in spite of these efforts, I'd add a "conditioner" that contained EDTA or another chelating agent. But if I had access to a clean roof that wasn't flat and tarred, I'd use rainwater, to be sure.

Modern rainbarrels. To get an overview of the kind of ready made fitted-out rainbarrel you could buy, or DIY projects for making one yourself, and the details of how to incorporate a filter in it or periodically to clean it out, King County WA maintains a web page "Rain barrel information and sources" that would make a good starting point. Or you could just run a search "rain+barrel" through www.google.com.The state of California and progressive cities like Portland OR will actually reimburse a part of your expenses for each downspout you divert into a rainbarrel. If you were building a new house in Bermuda or the U.S. Virgin Islands, you'd have to include a rainwater cistern. In the U.S. there are over 200,000 cisterns in use. Naturally, rainwater may not supply all your aquarium needs, but then I'm not suggesting you take yourself off the watermains grid entirely, either. Being a little Green today doesn't make you a Socialist or anything.

Using rainwater or those expensive high-tech water-softening alternatives--— distillation and reverse osmosis--— won't directly affect the pH. These moves reduce the buffering however, so that pH will become less stable in the softened water, just as it is in naturally soft water. You can't have it both ways: soft water is inherently less stable.

A green thought. Gallon for gallon, reverse osmosis technology creates several gallons of wastewater for each gallon of desirable filtrate. What will our great-grandchildren say, when they hear that, in order to obtain a gallon of water stripped of its mineral solutes, we were flushing into the sewer five or six gallons of filtered, pre-treated, chlorinated drinking water--— rather than trap and filter rainwater? That we saw nothing in this technology to make us uneasy? (When they hear that we were even flushing our toilets with purified drinking water, they'll figure we were pretty benighted anyway.)

Partial softening by boiling. Some low-tech water-softening can be achieved on a small scale simply by boiling. (Ingenuity and exertion so often can substitute for cash.) If you are wary of rainfall and currently saving up towards your costly reverse-osmosis installation, you can still have partially softened water, as long as all you need is a few gallons. Just boil tapwater in a large aluminum or enamel pot, the one you use for spaghetti or for a crab boil. As the water heats up, the carbonic acid (H₂CO₃) formed by dissolved carbon dioxide is driven off. Chemical balance is maintained as dissolved bicarbonate breaks down into CO₂ and carbonates (e.g. CaCO₃). Carbonate isn't soluble like bicarbonate, so it precipitates out: it's the white deposit that coats your kettle. (In a similar reaction, Ca and Mg carbonates can precipitate as a gritty white coating on your submerged aquarium heater.) Let the boiled water settle for a while, then siphon it off into your storage container. If you were to leave the water in contact with the carbonate deposits long enough, atmospheric carbon dioxide would slowly diffuse into the depleted water, where it would dissociate into carbonic acid and begin to reverse the process, to re-establish the former equilibrium. That's the reason for drawing it off.

The hardness that can be precipitated by boiling is often called the "bicarbonate" or "temporary" hardness. Boiling will not eliminate the other GH (General Hardness) components, such as calcium and magnesium chlorides and sulfates, nor will it affect nitrates, phosphates, silicates or heavy metals. This boiled water will be utterly depleted of oxygen as it is, so you'll need to run an airhose in it for a day or so, to diffuse oxygen into it again, before it's ready to mix with aquarium water.

Here's a good tale that's apropos: A fishkeeper with extremely alkaline well water (240 ppm KH) found that when his malfunctioning heater sent temperatures soaring over 85^oF, calcium carbonate precipitated onto plants and every inanimate object in his tank. After the disaster, KH was found to have dropped to 180 ppm! My hunch was that apparently the heat was enough to drive off sufficient carbon dioxide, so that the dissolved bicarbonates broke down into CO₂ and insoluble carbonates. This misfortune could only be an issue at extreme "KH" levels, but it suggested to me that he could routinely achieve some softening without actually boiling his extremely alkaline water, just with a heater in a 55 gallon container. I wish one of you with sufficiently hard water would do some simple experimenting and e-mail me about the results you get.

Soft water with high pH can be an obscure headache for some, most commonly fishkeepers living in areas where municipal water originates as groundwater drawn from limestone aquifers. In such waters, when dissolved calcium and magnesium exceeds about 150 ppm, municipal water boards may resort to a water-softening technique called the "lime/soda" method in order to precipitate some of the minerals, thus softening tapwater right at the plant. This treatment method is also being used to precipitate some dissolved organics that are harmless in themselves but combine with chlorine to form trihalomethanes. In the lime-soda treatment, first quicklime or its hydrated form is added to the water. Then caustic soda ash (sodium carbonate--— Na₂CO₃) is added to the water, to precipitate out the calcium and magnesium, largely as carbonates and hydroxides, which get left behind in large settling tanks. The resulting water is quite soft, but has a high pH. Further softening with porous cation and anion exchangers would be possible, but that technology is likely to be too expensive for most water utilities

What produces a high pH in water treated by the "lime/soda" method? The quicklime (calcium oxide--— CaO) used in this technique is produced in a kiln, where heat drives the CO₂ out of crushed coral, limestone or oystershell. The resulting caustic quicklime can be finely ground and slaked with water to form hydrated lime or calcium hydroxide, Ca(OH)₂, a fine powdery alkali that is strong enough to neutralize powerful acids. The slaked lime is also an efficient absorber of carbon dioxide, so tapwater treated by "lime/soda" softening generally arrives at the household tap still depleted in CO₂, which contributes to its high pH. Often fishkeepers with this artificially softened water that is combined with high pH find that the pH drops somewhat after 24 hours of curing, aerating it in a water butt. For the sake of pH stability in the aquarium, lime/soda softened water should always be separately cured before using it.

In areas with soft, naturally acidic water, utilities sometimes boost the pH, to reduce corrosion in the mains, by adding calcium hydroxide alone. This also can give tapwater with unusually high pH. On the whole, if tapwater has a pH >8.3 or so, you should expect that the water has been "limed."

In either kind of situation, a do-it-yourself CO₂ diffusion set-up in the water-curing bin could help restore the depleted CO₂, dropping the pH to more ordinary levels before you add it to the aquarium. An airline would speed the process.

Alternately, this is a situation where I think peat filtration would work to drop the pH, all the more effectively because the buffering is so low.

Peat filtration. Peat also softens water slightly, and the way it does this has the effect of reducing the pH. The complicated organic molecules of humic substances in peat bind the positively-charged calcium and magnesium ions and exchange them for positively-charged hydrogen ions. The more free H⁺ ions, the lower the pH, by definition.

Peat filtration is also a convenient source of soluble polyphenols and humic substances. Peatwater is one of my substitutes for handy "blackwater extracts." Others are leaf litter, Osmunda fiber, coconut shells, even a used green tea bag in the filter.

What kind of peat to use? Don't be dissuaded from buying plain inexpensive unprocessed Sphagnum peat moss that comes packaged in small compressed bales at your garden center. Better check the labelling to make sure it doesn't have any fertilizers or other additives contaminating it. You can be sure that any added ingredients will be featured in large letters on the packaging. There is no difference between rough-cut garden-center peat with a few twigs in it and sieved peat compressed into tidy little pellets and packaged for the aquarium hobby-- except in the price. If you purchase aquarium-processed peat from Fluval, Eheim, Aquatronics, or Marc Weiss you will not have any problems with too much or too little peat. These compressed peat pellets are also sieved to eliminate all but the larger granular or fibrous materials, which won't blow around in your water.

I've always used peat right in a bag in the filter, but I have very soft water and smallish tanks, and I can measure the amount of peat I need in soupspoons. You might tie up a bag made from the toe of a nylon stocking, set "upstream" of the main particulate filter, which will trap any stray peat fibers.

Pre-wet the peat. When it's bone dry, peat can resist wetting; so, pouring boiling water once over the peat first helps wet it and control it. If you have any hesitation about the peat permanently in the filter becoming too strong, steep it overnight in boiling water and add the "tea" instead. In general, stability of the pH is more important to fish than any particular pH value. Don't rush too fast to add more peat: use a little, then test it and wait a day. You should avoid making changes greater than 0.2pH in a day. Keep a log.

I keep moist peat in a big plastic funnel, set over a large jug. When I first fill the funnel with briefly-boiled peat moss, the initial couple of jugfuls dribble through a little too quickly, and I have to give them another pass. But soon the peat fibers swell and settle down and clog up, so that peat-water dribbles and drips through overnight. It's enough golden peatwater for my apartment-scale needs. I only need to pour boiling water over the peat one first time and steep it like tea. After that I can just keep it wet by passing water through it every few days.

By the way, maybe you should avoid aluminum pots if you're actually boiling peat. The peat's acidity could release toxic aluminum ions. Stainless steel is safe. Pyrex or enamel pots are even better choices.

"Spent" peat. Peat is "spent" when it has adsorbed as much Ca and Mg as it's going to. Peat should be renewed if the pH begins a gradual increase over a week or so, rising toward your normal pH value. This is a sign that the peat's "exhausted" and should be put out to pasture. Your water tests will give you an idea when the peat filter isn't working any more. Then you'll just get the feel of it. But the color of the water isn't a dependable indication. You may find that even clear peat filtrate is still softened enough for your purposes. Try to use your "spent" peat in repotting houseplants or in your garden, or culture Grindal worms in it. If you're setting up a new aquarium, spent peat, with its acidity neutralized, can also be an excellent source of humus in your substrate. In the garden it's a great conditioner, adding texture and water-retaining humus to any soil, and its adsorbed calcium and magnesium help counteract its acidity.

Use a low-ash activated carbon to remove any resulting tannic color from the water. The lower ash carbon does not affect the pH. But don't combine carbon and peat filtration at the same time: the two are working at cross purposes. Alternate them, if the amber color of tannins that peat releases into the water offends you. Let the peat do all the softening it can, and then remove tannins with carbon.

Burningham's Peat Bucket. In highly buffered, alkaline water you won't get any appreciable softening with spoonfuls of peat in the filter. You need a whole bucket of peat.

Englishman Mark Burningham really takes the bull by the horns. His peat filtration technique doesn't measure out mini-pellets of specially-packaged sieved peat one tablespoon at a time. He's taken a thoroughly rinsed new five-gallon plastic bucket and drilled a hole in the bottom, about a centimeter in diameter. Then he's covered the base of the bucket in an inch-or-so layer of filter floss. Over it he adds ordinary unadulterated baled peat moss from a garden supply center, enough to fill the bucket three-quarters full. He sets the bucket on a pair of wood slats over a large drum and pours enough boiling water through one time, to just wet the peat. He lets the resulting peat tea drain off into the garden, but if you wanted the tannin and didn't plan to use the water right away, I'd think it was fine for the aquarium.

Then Mark just keeps refilling the bucket whenever he thinks of it. His fuller description, with photos is at his site

Tim Bo has written a full account of the uses of peat water in the aquarium.


Peat filtration beds in wastewater management. Today's new wastewater technology has often been adapted for tomorrow's aquarium filtration, so you might be interested in the details of peat filtration beds. The way peat fibers behave in the domestic wastewater management field, where they're being used as a kind of filtering substrate, has been well explored recently, for example at a 1998 symposium at North Carolina State University, with articles that touch on peat filters in wastewater management. Beds of compressed peat, lightly buried in concrete or plastic containers, have been used lately to treat domestic wastewater effluent as it leaves septic tank systems. Two rival companies have been particularly involved in this technology; one, the Irish national peat board, Bord na Mona, call their system "Puraflo;" the other, a Maine-based company, touts its rival "Envira-pure" system.

Ion-exchange water-softening resins. In common household system water-softening systems, ion-exchange resins swap two sodium or potassium ions for each divalent calcium or magnesium ion they take up. The resin beads are recharged in brine. As a general rule, if the ion-exchange resin is "re-chargable" in salt brine, then it's putting sodium ions into your water. Good for lathering your shampoo, but not for aquarium work. Even if it is for sale at the LFS. Some aquarists dismiss the effects of this added sodium. But there is a better cation than sodium for recharging ion-exchange resins: potassium.

Recharging with potassium chloride. Though it is more expensive than common rock salt, potassium chloride is just as effective at recharging ion-exchange resins, according to a Cargill Salt website FAQ concerning water softening. The Q&A in question goes like this:

"What is the difference between sodium (chloride) and potassium (chloride)?"

"Both do the same job. They replace calcium and magnesium on the softener resin during the regeneration process. When you use sodium chloride, sodium will be added to the soft water during use and when you use potassium chloride, potassium will be added to the soft water. People whose physicians have advised them to eliminate sources of sodium from their drinking water normally use potassium chloride. In some people who have kidney or other renal problems, potassium can aggravate those problems. Most healthy people(>97%) can use sodium chloride without trouble and sodium chloride is less expensive. If you have any questions, consult your physician."

Biological water-softening processes. Plants take up calcium and magnesium in building and maintaining their structures, and they store them, thus functioning ecologically as a reservoir for these elements, like a standing forest that locks up carbon for a time. When plant stems and leaves are pruned or siphoned out, they are "exported" from your aquarium ecosystem. On the other hand, if they soften and disintegrate within the aquarium, their molecular building blocks remain part of the system and get recycled.

Snail shells similarly store calcium, but this gets returned to the system if the snail dies and you don't remove the shell. But when you flush Melanoides snails out of your filter medium and discard them, you are also "exporting" from your closed system the calcium represented in their shells. Of course, on a practical level, this isn't really a terrific way to achieve any notable water softening. So you may snicker. But the principle of "exporting" from the system is important to keep in mind, because in the end, "exporting" the ions like Ca and Mg that make water "hard" is the only way of permanently softening water.