Because most fish farms /breeding facilities are a circuit(water goes around in a circuit) you must test the water. there are tests you should do regular and those not so much, ammonia is the one you must do, to much in the water and you will have dead fish, as you can see above my ammonia level is getting high not dangerous yet but it has the potential to be. the test kit in the right upper corner are available in all pet shops .If your ammonia gets to high stop feeding the fish and add clean water.