What are acceptable water testing levels for pH, ammonia, nitrates, nitrites, oxygen?

pH: Koi are pretty tolerant fish to steady state acid/base conditions. pH becomes a problem when it changes rapidly and as the water heats up in summer. Koi do best at 7.5 (neutral) but will tolerate 0.75 plus or minus this as long as it is what they are used to. High pH (alkaline) conditions makes ammonia more toxic. The tendency of some koi shows to freely bubble their vats blows off CO2 and raises pH. At that point, even minor increases in ammonia levels can cause big problems with the fish. More of a problem, especially in bare liner ponds with high fish loads, is loss of alkalinity.

Alkalinity is, very simply, the measure of the pond's ability to keep its pH stable as conditions change. It is measured in ppm and ideal is around 100 ppm with a range of around 40 ppm in either direction. It depends on the presence of salts of various metals, primarily carbonate salts, which interact with the acid produced by the fish and the activity of the bacteria in the pond and the biofilter. This interaction smooths out the peaks and valleys of alkalinity and acidity that are natural pond water mechanics, and substantially lessen the stress on the fish and filter.

Low levels of alkalinity can lead to a situation in the pond known as a "pH crash". In this situation, (usually found in the above-mentioned bare-liner ponds with high fish loads), alkalinity drops below the level needed to stabilize pH, and the continued bioactivity of the pond causes rapid increases of acidity (lowered pH), often to levels as low as 6.0 or less. At these pH levels, the bacterial population in the biofilter stops working and then dies, leaving the fish to go on producing ammonia. The only half-good thing about this situation is that at low pH, ammonia is ionized, and relatively non-toxic. The danger to your pond occurs when you correct the alkalinity and pH before you remove the ammonia. Under those conditions, the ammonia returns to the non-ionized state and kills your fish.

Amquel will take care of the ammonia, just make sure you allow enough time to disperse it completely through your system before adding sodium bicarbonate to restore your alkalinity. Remember to take water samples frequently, and monitor ammonia first (see below) with a salicylate-based kit, then pH and alkalinity as you treat. Longer-term solutions to this problem include "buffer biscuits" made from plaster of Paris, or some limestone rock in the pond's structure. Also useful are bags of oyster shell placed somewhere in your filter system. All are good sources of carbonates, and are readily available.

Ammonia: Any persistent level of ammonia is bad, indicating too many fish, too little filter or not enough water. Ammonia spikes occur after rainstorms and during spawning, as well as early in the season when the fish wake up before the filter does. The more you feed (especially the high-protein growth foods) the more ammonia your fish will produce. Chemical treatments, especially formaldehyde and other disinfectants (Chlorine in tap water!) can kill off your bacterial population, crash your filter and leave you with ammonia galore and sick fish, as can natural loss of alkalinity (see above). Levels of .05 ppm are acceptable as long as they are recognized and dealt with, either by cutting down on feeding, lowering the population of critters or improving biofiltration. Sustained levels of 1.0 or above definitely indicate a real problem that needs a major fix. This is even more urgent if your pH and temperature are high. Your biofiltration setup's bacteria take ammonia to...

Nitrite: Also toxic, indeed more toxic than ammonia, regardless of pH. Even low levels (.05) are cause for alarm if sustained. This usually indicates overloading or a seriously sick filter. In Spring, nitrite spikes are a common problem because the bacteria that takes ammonia to nitrite grows much more quickly than the species that converts nitrite to nitrate. This causes an approximate two-week gap in your pond's ability to cope with your desire to feed all those hungry mouths poking out of the water. High nitrite levels in the pond cause "brown blood disease" in koi, very similar to carbon monoxide poisoning in mammals. The nitrite binds to fish hemoglobin and prevent it from loading oxygen, at which point the fish die. Salt will, to some degree, prevent this problem. (See the question about salt and the salt formula.)

Nitrate: Nontoxic but the best fertilizer in the world. Got green water? Got hair algae? This means you have a good biofilter and lots of nitrate. Plant plenty of pond marginals and water lilies to sop up the nitrate and your algae will go away.

Oxygen: Koi require levels of at least 7.0 PPM to remain comfortable. Cold water (38 degrees) can hold as much as 13 PPM As your pond warms up in the summer, the water can hold less. For this reason, most experienced ponders have at least one airstone running and many have installed venturis both to move the water and to inject air during the depths of the summer.

See also Who's on pHirst?

(Answer courtesy of Bob Passovoy)