Monitoring the quality of your pond water with a pond test kit is an essential part of pond fish keeping. Failure to do so could result in fish deaths that could be prevented by the use of a simple pond water test kit.
The four main chemicals to check for are Ammonia, Nitrite, PH and Oxygen. You can obtain testing kits for these elements either individually or in packages that contain most of the vital tests. They are also available as either liquids or litmus paper type like tests. The litmus paper type tests have the advantage of simplicity where several different tests can be carried out in one dip whilst the liquid tests are usually more economical.
Ammonia NO3 & Ammonium NO4+ (Ammonium NO4+ is relatively harmless but changes at high PH) is the most lethal of the chemicals you will need to test for and it is important to keep it at a reading of virtually zero. Ammonia is excreted by fish and also expelled as they breath. It is also produced by decay when dead plants, animals or fish are left in the pond or when food remains un-eaten, so it is important to remove these.
Nitrite NO2 is another dangerous chemical being the second stage in the nitrogen cycle. Both Nitrite and Ammonia are most likely to show up in the first few weeks of setting up a new pond, before beneficial bacteria have had a chance to colonise your ponds filter (please see Nitrogen cycle for an explanation).
PH is the measure of the pond waters acidity or alkalinity and there are two reasons why it should be tested. As the PH of your pond water increases Ammonia levels becomes much more toxic changing from Ammonium to Ammonia. Secondly fish will gradually acclimatise to the naturally changing PH levels in your pond. Without regular partial water changes decaying plants and the effects of the fishes respiration will slowly lower the ponds PH making it more acidic. This slow change will have little effect on your fish in your pond as it is very gradual and happens over a long period of time. However if you introduce new fish the PH could differ wildly from their previous home and the change could be enough to shock causing them to become ill and die, often weeks after moving them.
This is the natural conversion of toxic Ammonia into non harmful Nitrate by de-nitrifying bacteria. It takes several weeks for these bacteria to build up to levels where they can break down the toxins quickly enough to neutralise the filtered water. It is for this reason that you not put to many fish in a pond in the early days and that feeding must be kept minimal. Fish are not like cats and dogs and will not suffer unduly by not being fed for a week or two but overfeed them especially in the early days of your pond and you could end up poisoning them.