Algae: limiting nutrients
Nutrient control. "Algal control is all nutrient control." If your interest in algae just at this moment is basically limited to how to control it, Jeff Dietsch has some very good advice: "Algal control is all nutrient control," Dietsch warns, and though you wouldn't expect to find a simple magic cure for algae, Jeff offers a basic truth that is more helpful in the final analysis: a bloom of algae, like any booming population of one multiplying organism that overwhelms an ecosystem, is a symptom of imbalance in the system. So begin by checking out Jeff and Cassi Dietsch's fifteen helpful tips towards algae-free plant tanks at The Cassi Dietsch Zoo; they point out that there's not one answer to an algal plague.
The major nutrients of algae are nitrogen, phosphate and potassium, the same macronutrients required by plants. In their competition with algae, the vascular plants have an edge: they have means of temporarily storing excess nutrients, whereas algae are hampered if a nutrient is unavailable even for a brief period of time. This is the gist of controlling algae by limiting one essential nutrient. The 19th-century German chemist Justus Liebig first explored and popularized this observation that growth of plants is limited, not by the total nutrients available, but by the single scarcest resource: this is the gist of Liebig's "law of the minimum".
Paul L. Sears and Kevin Conlin started the discussion; their technique limits phosphate indirectly in planted aquaria by dosing with potassium. The famous "Sears-Conlin article" that people may mention, titled "Control of algae in planted aquaria", written in 1996, is archived at The Krib, where there is a storehouse of algal discussion. Refinements to the "Sears-Conlin" approach resulted in Paul Sears' further article "Control of algae in planted aquaria", which has a CO2 calculator, assessing the available carbon dioxide based on the KH of your water.
Limiting phosphate. The Sears-Conlin technique deserves a closer inspection. Though one phase of algal control may be nutrient control, not all nutrients can be effectively brought under control. It's not practical to limit nitrogen, for instance, what with the fish in the aquarium giving off ammonia, and all the decay processes generating more ammonia.
So, what about limiting phosphate? suggest Sears and Conlin. Why can't algae be repressed by lowered phosphate levels? A critic of making PO4 the limiting factor, however, might instance the role of alkaline phosphatase. Though the aquarium's plentiful reserves of organic phosphate generally are unavailable to algae and plants, when mineralized orthophosphate becomes scarce, some algae are able to scavenge PO4 groups from organic phosphates by using a metabolically costly enzyme, alkaline phosphatase. Alkaline phosphatase, which strips phosphate from many organic molecules ("dephosphorylation" the process is called), is produced in relatively large quantities by planktonic algae in response to low concentration of orthophosphate, the mineralized form of phosphate that all photosynthesizers use. It's a negative-feedback buffering system: the lower the orthophosphate levels, the more phosphatase is produced. In fact alkaline phosphatase activity is routinely taken by scientists as an indicator of PO4 limitation in phytoplankton communities.
Algae may be quicker and more apt than the "higher" plants to take up phosphate, but they have no mechanism for storing it. So over a period of time (be patient!) the vascular plants will scavenge phosphate and starve algae, the Sears-Conlin technique goes.
Though phosphates are part of the molecular structure of proteins, phospholipids and other essentials of fish nutrition, PO4 levels are particularly high in bones; the major source of phosphate introduced in your aquarium is the fish meal in flake feed, unless you're supplementing it with a phosphate-based pH buffer. So you might want to reduce the amount of flake feed in your fishes' diet, or switch to one of the new lower-phosphate flakes, one with a high protein-to-phosphate ratio. Frozen brine shrimp and frozen daphnia don't contribute elevated levels of phosphate; worms are boneless too. Addressing his fellow reefkeepers, Ron Shimek analyzed the nutritional content of a range of prepared foods for marine organisms ("Necessary nutrition, foods and supplements: a preliminary investigation") , and Randy Holmes-Farley calculated the phosphorus/protein ratio at Reef Central forums, 16 September 2008; nothing like this has been done for freshwater fish feed.
Phosphate-adsorbing granules can also be added to your chemical filtration media. The PO4 that they trap on their surfaces won't get rereleased into the water.
Dosing with potassium. An indirect way to assist in this process is to make sure that the third macronutrient, potassium, is always available. Potassium is ordinarily the macronutrient in shortest supply in the aquarium, though not in unpolluted natural waters, it appears. Adding potassium encourages vascular plants to scavenge more of the phosphate that algae need, as long as your lighting is sufficiently strong to encourage growth rates.
Fertilizing. The upshot: what I had been doing in my aquaria was to add potassium sulfate to the make-up water, at a dosage of 0.5ml/gallon. I did this at intervals, so that the potassium was delivered in pulses. It worked pretty well for me. If I were to dose fertilizer directly to the tanks, I could overestimate the amounts plants were using and build up unhealthy levels. That's why I dose the make-up water instead; that keeps me conscientious about water changes, too! The potassium sulfate fertilizer I was using also contains some iron. Since there's plenty of iron in my substrates already, and I don't want additional iron in the water column, I've switched to potassium chloride (KCl), which I get at a health-food store in the form of a salt substitute, "Nu-Salt." It also contains less than 1% cream of tartar (more potassium), silicon dioxide and "natural flavor." A literal pinch goes into my 6-gallon makeup water storage cans. I'm told I could also find potassium chloride more cheaply at Home Depot, among the water softener salts. Don't make a special trip.
Residual chloride questions. So the potassium is taken up by plants. Excellent. But what about that chloride ion? Couldn't Cl build up, if my partial water changes weren't enough to keep diluting it out? A few drops of sodium thiosulfate (de-chlorinator) resolves the issue, apparently. Seachem explain at their website that when harmless sodium thiosulfate — Na2S2O3 — gives false positive readings for ammonia, it is reacting with the chloride ion that is part of the test reagents, instead of the ammonium ion. After 24 hours, though, according to Seachem, the Na2S2O3 will have have reacted with chloride ions naturally found in water, and will no longer give such false-positive readings. So an amateur like me can use sodium thiosulfate to react with any surplus chloride. Jamie Johnson posted at the Aquatic-Plants Digest (23 March 2001) the relevant chemical reaction, which he expressed this way (with my translation):
Na2S2O3 + 4.Cl - + 5.H2O -> 2.NaHSO4 + 8.HCl.
(Translation: "Sodium thiosulfate plus 4 molecules of chloride plus 5 molecules of water give 2 molecules of sodium bisulfate plus 8 molecules of hydrochloric acid.")
And Na2S2O3 + 2.HCl -> 2.NaCl + H2O + S + SO2
(Translation: "And an additional molecule of sodium thiosulfate plus two molecules of that same hydrochloric acid give two molecules of sodium chloride plus a molecule of water, one of elemental sulfur and one of sulfur dioxide.")
Is any fertilizer absolutely necessary? On the whole, plants that aren't being dosed with additional CO2 rarely need additional fertilizing. Our aquaria are already "eutrophic" compared with natural tropical waters. It's often wise to get the system in balance first, without fertilizer. Once plants are growing modestly and algae are under control, then you can decide what kind of fertilizer you want to add. Go easy, especially on the micronutrients.
It's remarkable how many people posting their algae woes on the web forums did not discontinue their fertilizing program at the first sign of an algal problem. Still, you should know the recipe for PMDD ("Poor Man's Dosing Drops") which you'll find in the same algae-control archives at the Krib.