When cloudiness or "bloom" is a problem. Sometimes there can be a faint "bloom" to your water when the lights are on. It's as if the water were lit from within, with a hazy whitish glow.

You know it's not the microbubbles that can come out of solution when water under pressure is first released at the tap. They disperse within a minute.

You also know it's not the minute floating silty matter called "colloids", because your tank is thoroughly cycled and your filtration is excellent: this just isn't the "cloudy" water of a new tank. And you've put some in a white mug and looked at it in sunlight, and it doesn't have the telltale tint of "green water." I know hazy water can be a problem in a well-kept unplanted cichlid tank too.

The usual explanation for cloudy water is that it's bacterial and that it will "dissipate." I've had faint haze sometimes in planted tanks, but I'd attribute it to what I'd call "freshwater plankton," more than to bacteria.

Planktonic bacteria. There are comparatively few bacteria drifting free among the single-cell algae and euglenoids in the water. Bacteria colonize surfaces, as you'll know from reading about "biofilm" here. Some bacteria are attached to organic particulates in the water column, or a few may get dislodged, to blow around in the water, but the open-water population of planktonic bacteria isn't ordinarily dense enough to create haze, in spite of what you're often hearing.

Like all organisms, bacteria need carbon and energy sources and essential nutrients, which are too dispersed in aquarium water for it to carry many bacteria: a low population but a high diversity of kinds is what you'd expect to find in samples of your water. Give them a suitable nourishing surface, such as a plate of agar in a petri dish, and you can culture plenty of strains, from small founder populations.

More usually, it is the all-but-transparent bodies of freshwater plankton, scattering light that passes through them, that create the cloudy "bloom." Free-floating planktonic flagellates, ciliates, rotifers and the like, all dine on algae and euglenoids; without these constant grazers, the water might even green up. Copepods are at the top of this nutrient web of microscopic organisms, and you may actually be able to see some of the larger copepods by looking closely in a strong light.

This perfectly natural microscopic ecosystem forms the plankton of freshwaters. Call the planktonic population "infusoria," and you'll recognize it as a natural first food of the smallest fish fry.

Still, you don't want that haze in your show tank.

Clearing hazy water. Even in an otherwise unplanted tank, I'd recommend stronger light, less fish feed and a flourishing crop of floating duckweed, which you regularly harvest. Removing duckweed in a brine-shrimp net "exports" nutrients from the system. With duckweed flourishing, add about 3 ppm of potassium sulfate (Tetra's FloraPride for example) or potassium chloride to the make-up water. The potassium helps the duckweed scavenge phosphates, so the duckweed outcompetes the algae. Less algae makes the planktonic population that depends on algae crash in its turn, and your water turns crystal clear.

Hydrogen peroxide. If you're very cautious and conservative, it is possible to clarify slightly cloudy water with hydrogen peroxide. Like potassium permanganate, hydrogen peroxide oxidizes organic molecules when it comes in contact with them, releasing O₂ in the process. Healthy fishes are somewhat protected from the caustic effect of hydrogen peroxide by their slime coat. Their gills, as always, are the most vulnerable. Higher plants have protective cellulose in their cell walls and some, like Anubias, have the further protection of waxy cuticles. Not protected from H₂O₂'s reactive power: bacteria, including cyanobacteria, algae and many single-celled protists. So, if you can determine the safe dosage, hydrogen peroxide has the potential to reduce dissolved organics, raise the redox potential, clarify water by reducing the population of plankton organisms, and attack cyanobacteria and true algae.

On the other hand, if you artificially repress the population of the smallest planktonic grazers, like flagellates and the smaller of the ciliates , which keep bacterial populations in check, then could you be simply setting the stage for a temporary bloom of planktonic bacteria? A case in point in the Hudson River: when alien zebra mussels invaded the tidal lower Hudson, they filtered out flagellates to the extent that bacteria in the water have now stabilized at much higher levels than formerly obtained.

Still, you'd think the temptation would be irresistible to repackage hydrogen peroxide for the aquarium trade, as a wonder additive, at an inflated hobbyist price. And you'd be right.

See, you're getting to be a Skeptical Aquarist, too!

Dosing hydrogen peroxide. I use a capful, about 5cc, of standard 3% hydrogen peroxide per 20 net gallons. I only use hydrogen peroxide in a well-balanced mature planted aquarium, where the water has a faint haze, in spite of good filtration. If I were to use too much, or if I were to repeat the dose too soon, or if I used H₂O₂ unnecessarily in water that was already very low in organics, doubtless the fish would register distress, first by increased respiration. Why is this? Because if there aren't sufficient organic molecules or susceptible algal cells to expend its caustic energy on, hydrogen peroxide will remain active longer in the water and burn fishes' gills. The action of H₂O₂ should be largely spent within a quarter of an hour or so, but enough residual oxidative power may remain to frustrate your attempts to medicate for a day or more.

But before you resort to hydrogen peroxide, try clarifying the water by not feeding for three days, except perhaps with a little live food. If the water notably clears by the third day, it's a sign that you're in the habit of adding more flake feed than your particular system can absorb. Consider frozen food as another possible source of planktonic bloom. Defrosted frozen food that has not been rinsed adds quantities of nutrients in its juices; those nutrients may be supporting a planktonic population dense enough to creat a haze in the water.

Flocculants. Flocculants (an aspect of "Chemical Filtration") coagulate particulates and colloids, and planktonic algae too, and precipitate them out as loose, curdlike floc. I mention flocculants last, because hasty fishkeepers often rush to use them first, and because some major reservations apply.

Cloudiness caused by phosphate buffers. When phosphate buffers, such as Aquarium Pharmaceuticals' Proper pH, are used in hard water that has Ca and Mg levels well over 100 ppm, the phosphate can precipitate calcium and magnesium and adsorb them to itself, causing a haze. You should read A.P.'s Product TechSheet for their stabilizing pH phosphate buffers before you use these products.