Aquarium Pharmaceuticals Ammonia Test Kit
Ammonia, constantly released into the pond, is the number one killer of pond fish. Fast, easy and accurate, this test kit measures ammonia levels from 0 to 7 ppm. 130 tests per kit. This is useful in two circumstances. First, during the cycling phase, regular testing for ammonia will tell you when the first phase of the nitrogen cycle has completed. Second, should you have unexplained fish deaths, testing for ammonia verifies that your biological filter is (or is not) working correctly. Note that even in an established pond, the biological filter can sometimes weaken or fail outright.

Common causes include:

Not cleaning the filter regularly (water can't flow through a clogged filter, where the nitrifying bacteria reside
Naively adding fish medicines (antibiotics kill nitrifying bacteria (oops) as well as disease carrying ones)
Having too small a filter for the fish load

Ammonia levels are measured in ppm. At concentrations as low as .2-.5 ppm (for some fish), ammonia causes rapid death. Even at levels above 0.01-0.02 ppm, fish will be stressed. Common test kits don't register such low concentrations. Thus, test kits should NEVER detect ammonia in an established tank. If your test kit detects ANY ammonia, levels are too high and are stressing fish. Take corrective action immediately by changing water and identifying the source of the problem. Ammonia-neutralizing water additives are incompatible with most ammonia test kits. Water treated with ammonia-neutralizer will falsely test positive for ammonia, even when ammonia is not present. Test kits using the "Nessler'' method are known to give false readings under such conditions.