When your tank is established and you have fish in it you still need to do some maintenance on the tank.
While this can be a bit of a chore you will need to do it or your fish will die. This is part of the process of keeping pets and you will need to make sure you do it well to minimize the amount of work you have to put in. The right choice of filter will go a long way to helping you here as, as well as not overfeeding or leaving decaying food lying around.
To ensure things are not left to chance you can purchase test kits for things such as ammonia, nitrite, nitrate and PH.
If the fish are in an outdoor pond type environment it will, of course, not be necessary. However if you intend to breed or have very fancy types you may need to go to this extent. If you have these you can also test for chlorine, dissolved oxygen, phosphate and specific metals. You may need to test out the test kits to decide which ones are best for you. When beginning choose the ones with the clearest instructions and ease of use. Try to work out what they cost per test rather than the overall cost. This will prove cheaper in the long run.