Originally Posted by willieturnip
Crystal clear water and fish that "look good" means nothing as to how healthy they are.
Water tests aren't necessary once the tank is stable.
I would be interested in some clarification on these statements. It seems that you only have 2 ways to judge the health of a system. You either use visual observations, or you use a test kit. Yes?
I would personally encourage everyone to use every test they have. This includes your eyes and careful observation of every fish in the aquarium, at least a few times per week. Are the fish swimming well and, free of spots and torn fins? Are the eyes cloudy? Are the scales shiny and vibrant? Do the fish actively compete in the feeding area? All of these signs are visual and help to determine the health of a fish.
The same can be said of the water to some degree. Is there any cloudiness or yellow tint to the water? Cloudiness could signal a bacterial bloom, resulting from an ammonia spike. A yellow tint is possibly a sign of increased dissolved organic acids, tinting the water and suggesting that water changes are not being performed frequently enough. True, these are not perfect "tests", but they sure give you some warning signs on occasion that something could be wrong.
I personally also like to test the water, especially for pH and general hardness. These tests can provide you with some trends as to how the water is behaving between water changes. There have been occasions over the years where a rapid drop in hardness was an indication to me that something was wrong. Something as simple as a filter pad which went unchanged inside a canister filter could cause waste to decay, depleting the buffer system, lowering hardness, and ultimately causing pH to rapidly drop. A dead fish which has gone unnoticed could cause the same reaction. A simple hardness and pH test gives you confirmation that your water changing routine is effective, and that you have not overlooked a normal aspect of your aquarium maintenance routine.