Fish Disease can strike quick, even to the experienced keeper. Treatment is usually necessary right away, and alot of personal judgment comes into play as well. This can only be gained from experience. Here are some tips though:
Most fish diseases you’ll encounter will be will new arrivals, as once the fish has acclimated your chances are much better. Therefore quarantine is necessary to prevent outbreaks. The use of a UV sterilzer is also good for stopping outbreaks, and partially lessening the chance a fish will get disease. These are a "bonus item" for the average fish-only tank, unless one has really delicate Angelfish or Butterfly fish for example, but are definitely a great idea for a reef tank. This is because in a reef tank, the inverts cannot take the usual chemicals used, such as Copper, to treat the fish, and it will quickly kill them. Also, it is much harder to get a fish out of a reef tank than a fish-only tank, should it get disease. UV s also help get rid of some algae, especially the free floating kind that causes "green water" and the like. Try not to treat the whole tank regardless though, as this is costly, may affect filter performance, and will also stress out the other fish. Give the fish a simple sponge filter (no carbon at all) for aeration and general filtration in a 10-20 gallon tank (20 being better) and treat them in this tank. Remember, as this tank is not cycled more than likely, it is essential water changes are done at least a few times a week. Another good thing to do is add an old sponge to the tank, or possibly some live rock, to increase immediate filtration. Of course, the live rock will most likely die, depending on the medication used, so an old sponge may be the best filter to use for immediate partial biological filtration.
Prevention is the best cure. Good filters, protein skimmers, and regular maintenance (such as water changes of around 25% monthly, with gravel cleaning as well) will greatly reduce the chance of a fish getting disease. However if it does get disease, then ask your self this question, "Is the fish still eating, and is he still active?" If so, and it is a rather hardy fish (like a Damsel, Dottyback, Triggerfish, Lionfish, Hawkfish, etc.) then I would personally monitor the fish closely over the next few days and see if they "cure themselves." If it is a delicate fish, or a fish that is obviously in great distress, then action must be taken as soon as possible. It’s good to have medications on hand, just in case, as well.
If the fish is scratching against the glass, it is stressed. Maybe due to confinement or other reasons. This is not much to be worried about if it is eating still and looks good and colorful as normal. However, if it gets to the point where the scratching on the glass annoys you, then the fish is definitely majority stressed. If it gets to the point where he is trying to "rub the side of his skin off," or if you see his gills red, then treatment is needed for sure. Parasites near the gills will kill the fastest in my opinion (such as those on the pectoral fins coming out the front sides of the fish, that they mainly use to swim with). Good diet, water quality, tankmate compatibility, etc. will decrease stress in your tank. If the fish has white spots, this could be a variety of parasites, most likely Ick (Ich). If it has cottony white growths protruding from an area, it is most likely a fungus of some sort. These are two of the main types of diseases the average fish keeper is likely to see.
Treatment for disease varies with opinions, as most things do in the saltwater world. For Fungus, it depends on the fish, and some of the following stuff will work as well. For parasites:
Copper is well known as a cure-all in the saltwater hobby. It is readily available, and works great against parasites. However, some fish (Dwarf Angelfish and the Harquelin Tuskfish come to mind) can be very intolerant of medications, so be very careful with the dosing. A Copper test kit is needed, and recommended values (usually in PPM (parts per million) can be found out depending upon your fish.
Another known good treatment, for parasites or flukes for example, is a combination of formalin and malachite green. Malachite green is also known to work good for Fungus. Formalin, about a 35% yield is standard I think, and malachite green together have more of an impact than the sum of both alone. However, when dosing both, be very careful of the doses, as they will probably need to be cut down. Malachite Blue may also be recommended, depending on the disease, but the listed ones above are great in most cases.
Reverse water baths / dips. Another area of major controversy. The idea is that freshwater exerts a different pressure upon the fish, thereby stressing the fish, but not nearly as much as it stresses the parasites, which will then drop off. Needless to say, don’t put the bathing water in any tank. Get a bucket or small tank and mix it up with freshwater, or for the perfectionist--9 parts freshwater and 1 part saltwater. Make sure this water is very close to pH and temperature as well, or the fish may go into shock, and will at least be more stressed. Should you do a freshwater dip? Well they do help. For Fungus, no. Parasites, it’s up to you. I cannot recommend using this on delicate fish from what I’ve heard, however for the average fish it can be a very cheap, available (for sure), and effective tool to at least give the fish a fighting chance, if not kill the disease completely. THE MAX TIME IN FRESHWATER IS 2 minutes!! This is NOT like saltwater dips for freshwater fish (15 mins.). Watch the fish VERY closely while in the freshwater dip, as yes it does stress them alot. However it works, and many public aquariums use this.
One way to lower the chance of ick spreading, or infecting, is very simple. Lower your salinity to around 1.020 or 1.021. This will lower the chance of Ick spreading. You can keep it at this level always. If you have a Fish-Only tank, and you have sick fish, SLOWLY drop the level down to around 1.018. Also, keeping the temperatures a little warmer in the tank (76-78 degrees) also helps reduce the chance of disease spreading. Any specific or general questions can be directly to: witmer26@erols.com via email. Thanks for educating yourself, and good luck. Prevention is the best cure by far, and sometimes the only one. Keep up the water changes and it will save you alot of work in the long run :-)