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.

COMMON FISH DISEASES 

Tail & Fin Rot	The tail & fins rots away. The edges are jagged like its been bitted by something. In serious cases, the fins and tail are torn or split opened.  If the medication applied is effective, a black outline can be seen along the torn fins.
White spot	Fish body is covered with white pepper-like specks. Very common when temperature drops. This is as common as our common cold. Keep your water temperature constant.
Mouth Fungus	White growth on the mouth. Much like wearing white lipstick but worse as cotton-like growth can sometimes be seen from the wound.
Fish Lice & Anchor Worms (not a disease)	Usually they are introduced into the tank with live foods or plants. Although copper is the main ingredient used in treatment, you can also physically pulled them out with tweezers, and dap the wounds with "RID-ALL" Anti-Fungus.
Fungus	The white cotton-like growth is Not part of the fish fin. It is a secondary fungus infection normally due from injuries like fighting, or rough handling.
Pop Eye	Eyes swell out from its normal place and looks like it is about to pop out.
Swim Bladder Disease	The fish either floats at the water surface or sinks to the bottom when not in motion.
Dropsy	Abdominal area swells up and sometimes the scales protrude out. There is no known effective cure.
Hydra (pest)	This is a pest that may be introduced when you feed live foods. They are a danger to young frys only and post no threat to adult fishes. Click picture on the left for more information.