Water softening

Water softening involves reducing certain dissolved minerals in the water, though not all dissolved minerals contribute to hardness. Reducing the carbonates and phosphates will reduce the alkalinity or "buffering" that keeps the pH stable. Reducing the ions will lower the water's electrical conductivity. Diluting the total dissolved solids with distilled or reverse osmosis ("R/O") water is one way to achieve these objectives. Rainwater is a cheaper alternative. Peat filtration is also something to look into.
 
 
Rainwater. Let me begin with rainwater. Fifty years ago, clean rainwater was taken for granted as everyone's birthright, and fishkeepers spread plastic tarps to catch it. Since then, 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.
 
Pollutants in your rainfall. 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 (SO₂), lead and particulate matter, a.k.a. "soot"— which is largely inert black carbon.
 
So, then: 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. In very soft water like mine, with very little alkalinity, the result could slightly  lower the pH. Not really an aquarium problem, troublesome as the atmospheric acids may be in the environment as a whole. Ozone also quickly reacts, promiscuously 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, carbon soot and the volatile organic compounds, such as molecules of unburned fuel, benzene, and the like, most of them recalcitrant chemicals that can't be scavenged by common bacteria. How would you deal with these? Filtering out soot and other 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 need to be chelated, perhaps with peat or your EDTA-containing conditioner such as a "blackwater extract" — or a cooled cup of green tea. Or adsorb heavy metals to media in your chemical filtration.    Quite doable, yes?
 
Other pollutants. 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.
 
Atrazine. Some time ago (17 April 2002, New York Times), some fishkeepers were alarmed to hear from the US National Academy of Science that traces of atrazine, Syngenta's common weedkiller used by agribusiness, are now found, not just in runoff, and in 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, that's parts per billion). Atrazine causes frog cells to produce an enzyme that converts testosterone to estrogen: tadpoles exposed to atrazine in the lab developed multiple sex organs or were hermaphroditic, and testosterone levels in adult male frogs were decreased to levels more characteristic of females. Adult females, on the other hand, 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. 
 
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 former site maintained by the U.S. Geological Survey, which brought 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. The pH readings were later systematically dropped. There was 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 certainly 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 was a definite drawback for some aquarists, but  modern rainbarrels have a diverter, an  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: EDTA (ethylene diamine tetra acetic acid) is available in powder form from National Aquarium Pharmaceuticals, if you can trust yourself to prepare a stock solution that you then add by the teaspoonful to rainwater, remembering that the stuff is widely used to dissolve away limescale and will severely drop pH. 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, the rain barrel guide would make a good starting point. Or you could just google "rain+barrel".
 
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 be required 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.
 
Rainwater is free. Is it a cynical observation, or merely a realistic one, to note that there's no market incentive for any hobby professional to recommend rainwater for tropical fishes, don't you see? Just the Skeptical Aquarist, and the rainbarrel-makers. Instead, discussion centers round three widely recommended high-tech alternatives to toxic ol' rainwater: reverse-osmosis (r/o) water, distilled water or the use of ion-exchange filters. My own ignorance here is unblemished. How these work and the commercial products available along these lines are discussed by the knowledgeable at many bulletin boards and websites: start at The Krib.
 
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 tank  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, by 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 speedily restore the depleted CO₂, dropping the pH to more ordinary levels before you add it to the aquarium. An airline would speed the process. 
 
Peat filtration. 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 has its own page.) Pre-wetted sphagnum 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, one of my substitutes for handy but expensive "blackwater extracts,"   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.  Other sources for mild "blackwater" chemistry with somesimilar effects are leaf litter, Osmunda fiber, coconut shells, even a used green tea bag in the filter.
 
 
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.
 
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. Others tell you that household water softened by ion-exchange is never suited to aquariums. 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 (KCl) 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."
 
Another good archived article encouraging you to substitute potassium for sodium is "An Alternative to softening with sodium", from Pipeline. a newsletter of the National Small Flows Clearinghouse, Winter 2001.
 
When substituted for rock salt, potassium chloride works similarly to replace Ca and Mg ions. Potassium is more desirable than sodium: in a planted tank the additional potassium would be used by plants, so that there should be no build-up to be diluted by water changes. In fact, dosing with potassium is central to the "Sears-Conlin" method of controlling algae by limiting phosphate in planted tanks.
 
You can get potassium chloride in the form of salt substitutes such as "Nu-Salt."  Though I don't need to soften water here in New York, I don't understand why recharging ion-exchange resins with potassium chloride isn't perfectly standard aquarium practice.
 
Other ion-exchange resins suitable for aquariums work in a way similar to peat, by adsorbing calcium and magnesium ions, thus softening the water, and by releasing hydrogen ions in exchange. The additional hydrogen ions increase the water's acidity, and the pH value is consequently lower. This type of ion exchange does not increase levels of sodium or potassium in the water.
 
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.