Fishes normally tell you that they are under stress, or that something is wrong with the water - like the pH is out so that you can check if your filter is choked.
If your pH is very low, your discus becomes very light in colour. If your water is too alkaline, it becomes very dark. So when your fishes suddenly looses its colour or becomes very dark, something is wrong with your tank.  Their eyes turning white or hazy, loss of appetite, nervousness, etc, are signs that something is wroung with your tank.   Although they cannot speak, they do "communicate" with us.

Fishes show symptoms of sickness prior to actually getting sick. So if you solve these problems before they get too serious, you will not have to worry about diseases.

The telltale signs or Symptoms Of Imminent Problems: (this true for almost all the fishes)

Eyes have a white shade	Check pH, pH is probably on the extreme.
Fins clamped or closed	Check water condition & temperature
Fishes looses colour	Insufficient sunlight.
Tails & Fins have a jagged edge	Water too hard or alkaline, check activated carbon
Tails & Fin "blood-shot" esp. Goldfishes	Water condition is not in balance, too much salt and water maybe too hard
Excretion Floats - air trap in excretion	Constipation
Excretion hanging from the fish	Constipation
Fishes become timid, jumpy & nervous suddenly, hiding in the corner all the time	Water too hard or alkaline, too much salt, not enough hiding place, or bullied by others
Yellow residue at the water level	Water too hard, remove crush corals/shells
Fishes becoming very slimy	Too much salt or chemicals used
Fishes float at 30° along surface of water	Chlorine attack, or gills injured by chemicals or internal parasite
Suddenly refuses to eat	Sudden drop in pH, or temperature
Water have a smell	Check filter and replace activated carbon or zeolite
Body have spots, or blotches	Check filter, pH and replace activated carbon or zeolite. Too much salt may have been used; or attacked by other fishes
Fishes rocks its body without moving	Check temperature. Make sure you have a constant water temperature.