Chlorine & Chloramine Test Kits
Tap water contains chlorine or chloramine (chloramine is more aggressive but essentially the same thing) for sterilization purposes. Chlorine and chloramine are toxic to aquatic life and cause immense stress and damage. Use chlorine and chloramine test kits to test your tap water before setting up a tank or doing a water change - if you have either, use chlorine removers.
It is also a good idea to keep chlorine and chloramine test kits around in case you see signs of fish stress - chloramines can sometimes form in an aquarium, especially if you have problems with ammonia or pH levels, either of which need to be counteracted immediately.