The ideal water quality for tilapia is free of bacteria and nitrate. Bacteria are naturally present in water. They are normally harmless to fish, but if the water contains high levels of bacteria (such as when a reservoir or pond is over-fertilized), they can cause disease in fish.
Nitrate is a naturally occurring compound found in water that causes blue-green algae to grow in water containing too much nitrate. Blue-green algae are very toxic to fish and may even kill them. Nitrates can also be an issue in lakes, ponds and streams. Most water testing services will do this for you. If you don't want to go through the hassle of sending your water to a lab, there are kits available that you can use at home. You can also get kits at your local sporting goods store or bait shop.