For artificially constructed backyard ponds with pumps and filtration, preparing water for your first fishy residents means first treating it with a water conditioner that will remove both chlorine and chloramines, and bind and detoxify ammonia and heavy metals like lead that are frequently present in tap water or rain water. After your water has been conditioned, the most important thing to check is your water chemistry. The details will vary slightly according to the fish you’ll have, but generally you’re aiming for a pH of 7.0 and an absence of chlorine and chloramines. Next comes the nitrogen cycle. You can speed up the process of cycling your pond by adding a dose of beneficial bacteria and give your pond a couple of weeks to reach equilibrium, then run some more tests. Nitrites and Ammonia should be absolutely as close to zero as possible, but your fish can manage with levels up to 0.25 ppm
If you’ve got an existing pond where fish are mysteriously dying, you’ll need to do some detective work. Does the pond have an objectionable smell (rotten eggs, fishy etc.)? If so, you most likely have a problem with anaerobic bacteria and low oxygenation creating dead zones. Your best bet is to clean out your pond to remove the sludge and detritus that has collected at the bottom, perform a major water change, then add some bottom aeration to prevent the problem from recurring. Does your pond have a source of water other than the treated tap or rainwater you add manually? If so, you’ll need to investigate the quality of that water source - could there be toxins, heavy metals, pesticides, or even (heaven forbid) raw sewage contamination? Facing that kind of dire situation, you’ll need to consider how you can reroute the flow away from your pond. It may even be necessary to consult with an engineer to look at dikes, berms or other significant construction. Bacteria, parasites and other pathogens are also possible sources of repeated fish death. It can be useful to work with a certified pond professional who can test your water and even visit your pond to determine the source of the problem.
