This article is from the Beginning Fishkeeping FAQ, by Thomas Narten with numerous contributions by others.
There is a seemingly endless array of test kits for testing everything
from ammonia levels to phosphate levels. Does one really need to buy
them? The quick answer is no. It is quite possible to have a healthy
tank without ever buying a single test kit. However, test kits are
extremely useful at eliminating guesswork when something goes wrong
(e.g., fish appear stressed or die). In the following, we describe the
test kits that are most useful and the conditions under which they are
useful.
 
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