Medication overview: When dealing with fish diseases, first you have to identify what you're treating. There is no mediation that is effective against everything. Few fish medications are even good at treating a couple different pathogens. Get a good book on fish diseases and study it. Even then, you will most often have to make your best guess as to what fish medication to use. Also realize that it is more common than not, that you will be dealing with multiple disease problems. Most diseased fish are so weak that their immune system will not prevent attack from other opportunistic pathogens.
How to prepare: Before deciding to treat, first check the basic water parameters like temperature, pH, hardness, ammonia and nitrite levels. You don't want to stress fish with a medicine if the problem is something other than a pathogen. Also check to see that the filter is not clogged or in need of cleaning. Be sure there is good water circulation which is necessary for optimum oxygen levels. Look for fish that are bullying others. There may also be an ornament or gravel that is leaching a toxin. Make sure they're aquarium safe. Before treating, do a water change. This is often the most effective treatment and sometimes the problem goes away with increased water changes alone.
Deciding what to buy: After you have a problem is never the best time. Quick action is almost always very important, therefore, have some medications on hand and have a system set up so you can implement the course of action quickly. First, have a hospital tank (or two) ready with a biologically active filter. Use this specifically for treating fish and nothing else. Use the smallest tank appropriate for the fish you're treating. It will save on medication, make the frequent water changes easier and is less of a pain to set up and take down. Several medications will stain the silicone, so there is no sense using show tanks to medicate fish in. Malachite green, methylene blue and copper sulfate are some of the worst at leaving stains. Many medications will also kill plants and invertebrates. A hospital tank will treat only the fish. The hospital tank should have direct light over it. This keeps the fish more calm and bacteria grow faster under bright light. Light will also degrade some medications. When done with the treatment, sterilize the tank and filter for the next time you need it. After sterilization, the filter can be added to a healthy tank to re-seed the bacteria for the next use.
Many medications will harm the nitrifying bacteria in your filter, so have an extra filter in healthy tank, ready to be put into the hospital tank if needed. Nitrifying bacteria are gram positive aerobic bacteria, so if you use a medication like erythromycin, the filter will be killed. Some broad spectrum antibiotics like Kanamycin Sulfate are more effective against gram negative bacteria, but will kill some gram positive aerobic bacteria, like nitrifying bacteria, when used heavily.
Medicated foods will protect your filter, but when using a medicated food, it is critical that the fish are accustomed to eating this same exact food without the medications. Medicated food only works if the fish consume a particular amount of food. In general the medicated foods are formulated for a fish that is eating 2-3% of it's body weight in food each day.
Treatment Guidelines: If you suspect an internal parasite - high heat is beneficial. The parasites don't like it and some are actually killed by it. In addition, the fish's immune system is boosted at higher temps. 95-96 F is the most effective heat range. Most tropical fish can easily handle this if the tank is well aerated and the temperature is raised gradually over a 24 hr period. If in doubt, simply carefully monitor them while raising the temperature.
If you think it's a bacterial problem, high temperature is harmful. Bacterial thrive in the 80's and 90's, so it's best to treat in the low 70's. At this temperature, they won't eat much, so feed very lightly. In fact, if you suspect an internal bacterial problem, food is not going to help. Unless it's an antibiotic food, the fish are better off not eating. If fish do not respond to the antibiotic within 4 days, discontinue it and try another. If they respond, do not stop treatment. The full course must be given or they will be re-infected and become worse than before. Be aware - many manufacturers recommend a dose that is inadequate for a cure and many also have "cut" formulas that have a bulk inert ingredient added, which further dilutes the effectiveness of the medication. Pay attention to the purity of the medication.
Always choose a medicated food for an internal problem if you can find one with the proper medication and the fish is eating well. It is more effective, less harmful to the environment, saves the bio filter and is a less expensive way to treat. You can make your own medicated foods by combining the food and medication with gelatin to hold it together. Just soaking a dry food in medication is not very effective. The medication dissolves or disappears into the tanks water within seconds and the dose cannot be controlled.
Expired medications: Most medications will retain over 90% of their effectiveness well past their expiration date. The dates mandated by the government are more than just a little conservative. There are a couple, like Trichlorfon (dylox) and Oxytetracycline Hydrochloride that can become toxic when they get too old, but even they are probably good for well over a year past the expiration date. Keep them cool and dry for maximum shelf life and effectiveness. Do not freeze.
Overview of common bacterial types: