Common problems in new tanks and ponds
A very common problem, particularly for those new to fish keeping, is ammonia or nitrite poisoning. Unfortunately in many cases by the time it becomes apparent that a problem exists the fish are often dying or seriously ill.
Ammonia and nitrification
Central to the principle of filtration and fish keeping is the fact that fish are continually polluting the water they live in. These pollutants can very quickly build up to dangerous levels. A major pollutant is ammonia - which fish are continually excreting from their gills. Fortunately there are naturally occurring bacteria that "feed" on ammonia and convert it to less toxic nitrite. There are other naturally occurring bacteria that "feed" on the nitrite and convert it to harmless nitrate. So in a mature environment the ammonia rarely builds up to a dangerous level as it is quickly converted to harmless nitrate in this biological two-stage process called nitrification.
So far so good, the problem is that these bacteria take some time (4 - 8 weeks) to colonize both the tank / pond and filter - during which time the levels of ammonia and/or nitrite can very easily reach toxic levels - a condition called New Pond / Tank Syndrome. During this potentially dangerous period the water has to be monitored and managed.
These topics - ammonia, nitrite, nitrification and filtration - are explained in more detail on the related pages. A must read for all beginners!
Obviously the more fish that are in the system, the more ammonia will be produced and therefore the greater the potential danger. So, fish should be introduced gradually - just one or two at a time - allowing the system to adapt to the increased ammonia production. For example, if the tank already has two fish, adding another two will double the ammonia production and it may take a week or two for water quality to normalize. If a a few weeks later we add another two, this time the ammonia production will only increase by 50% as we already have four fish - so the filter will adapt fairly quickly. So by adding stock slowly we give the filter chance to catch up with the increased ammonia load.
Testing for toxins
The only way to tell whether the ammonia / nitrite levels are acceptable is to regularly test the water. Ammonia and nitrite test kits can be bought fairly cheaply and testing should be done from day one. Initially it will be necessary to test 2-3 times weekly until such times that the system settles down. Testing should be stepped up again each time new fish are added. It is best to keep a record of the results for future reference.