Close observation may save your fish - FIRST AIDS. Very few times the onset of a disease is so rapid and the symptoms so severe leaving you with nothing to do. Usually, the symptoms are there only we are too busy or not experienced enough to see them. Most people will recommend to you to isolate the diseased fish and treat it separately. There are good reasons for this : less medication is required and you do not stress the rest of the fish. However, if the dose to be used is not very high, I prefer to treat the whole of the tank. If a microorganism is in the tank it is going to attack a second fish sooner or later. Even if I isolate the fish (intending to use my famous overdose schemes) I usually treat the tank as well with a lower concentration of the SAME medication.
Signs that should immediately alert you are the following. If a particular sign can be attributed to something else, this option is examined, too.
1.Loss of appetite. You surely know your fish and the way they attack their food. If a fish loses its appetite or refrains from eating it is an alerting sign. Africans are not very particular on their food (with some exceptions meant for the most advanced keepers) so if they do not eat, something is wrong. You should isolate this fish because there are many medications that could be used and is it is not wise to treat the whole tank with all these regiments (an exception to my rule). I usually start with a general remedy and medicated food and watch very closely to see if other symptoms are developing that will help me to be more specific in my treatment. If the fish is a female it could be refraining from eating because it is carrying eggs. In this case you should be able to see (from time to time) a "chewing" movement which is a clear indication of carrying eggs.
2. Change of behavior. A fish that used to be very aggressive is suddenly avoiding fights and is trying to be as far as possible from possible trouble-makers. A fish that is lying at the bottom is also not a good sign. Watch it closely for a while. If the other fishes are moving around but one particular fish is just lying on the bottom it is a sign of disease. Scratching on the decoration could be an indication of a problem. This is common practice by African cichlids (it is like defining the territories) but it should happen once in a while by various species. If a particular fish is scratching very often then it could be a sign of parasites (a treatment with cupric sulfate + metronidazole could solve this problem).
3. Change of swimming behavior. Usually a fish can't swim as it should. It will swim at various angles or will clearly seem "de-stabilized". Sometimes you can see that the fish when it stops it sinks to the bottom (usually head down). It could also have erected scales. All these are signs to be very much afraid of since most often these are the signs of dropsy or Malawi bloat (a general bacterial infection). The fish should be isolated and treated with antibiotics (minocycline, oxytetracycline etc.). Do not feed for a couple of days and then only medicated food should be administered (if you are lucky enough the fish will still be in eating condition). Remember that tetracyclines are photosensitive. The tank should have no lights for the first couple of days and a blanket in front of the glass.
4. Clear signs on the skin or eyes. All sort of spots, holes, cysts or "hairy" things on the skin, eyes or mouth of your fish are signs of infection. The first thing to do is to make sure that it is not a removed or lost scale because of a fight. If it isn't, then you should check a book, determine what this spot looks like and treat accordingly. If more than one fish are infected at the same time there is no reason to isolate these fishes. It is highly recommended that you treat the whole of the tank instead.
5. The wastes of the fish look transparent and / or it takes too long to be separated from the fish. A sign of internal infection. Treat the whole tank again. Since the wastes already carry the causing microorganism which is therefore liberated in your tank you should treat the whole of the tank.
6. Many fishes gasping for air at the water surface. This could be because of many reasons but it is usually not associated with a disease but rather with the water conditions - notably the following :
Very little oxygen is dissolved in the water in which case you should
Increase aeration and surface water movement
Decrease the water temperature by a couple of degrees.
Too much carbon dioxide in the water - reactions :
Stop carbon dioxide injection immediately
Start heavy aeration
Increase surface water movement
Check your filters for clogging
The water temperature is too high (upper 80's in Fahrenheit, more than 30 in Celsius) - reactions
Open the tank canopy immediately
Reduce the number of light tubes or bulbs which are on
Reduce the light period
Add a ventilator or two at one side of the tank along the long axis
Increase water movement
Introduce a frozen bottle of water - do not add the ice directly in the tank
Make a massive water change
Buy a chiller if you have this kind of bucks
A high ammonia or nitrate level - use the relevant kits which you should already have at home - reactions:
Massive water change (75% or more, usually followed by another big water change next day).
Check filters for clogging
Introduce an Ammonia absorber
Add an ammonia removing chemical
Add some bacteria - if the problem is nitrates you could add some algae or live plants.
Check your tank for dead fish/invertebrates. A dead, decaying fish could be the reason.
Clean the gravel to be on the safe side.
If no signs correspond to your observation, see the Disease List.