BillD
Aquarium Advice Addict
It appears that the problem here is the ammonia in the tap water, probably due to the use of chloramine. Since there are 0 nitrites and the nitrate isn't unreasonable at 20, I would suggest checking the ammonia when the water change has been done, and a few hours later and then again a few hours after that. The ammonia should drop with each test if the filters are working. Water conditioners such as Prime don't actually neutralize the ammonia, but rather convert it to a non toxic form (ammonium), which will show on an ammonia test even though there is no toxicity. As far as how often a water change should be done, the nitrate number will tell you that. The lower the better, and keeping it below 20 should be your goal. In most cases, stocking levels, as recommended, are a rule of thumb. If you were willing to do the work and have adequate filtration, higher than usually recommended stocking levels are possible. An example of where this would happen regularly are breeder's tanks where very high concentrations of fish are kept to grow out and the overcrowding is compensated by large frequent water changes. However, this isn't practical with a show tank, where you are basically keeping a few pets. The breeder situation is more like farming.