Please keep in mind that an actual veterinary examination by an experienced vet is the best way to help any sick animal.
Unfortunately, diseases among fish are very common, and something every fishkeeper must deal with. In most cases, they can be treated with a course of antibiotics, or some other medicine, and your fish will be back to normal in no time. The best action you can take, however, is prevention. Here are some guidelines for keeping your fish happy and healthy. And in case you still wind up with a sick fish (it happens, and isn't always your fault), a link at the bottom of the page will take you to a symptom and treatment chart of many fish illnesses.
Quarentine new fish before adding them to an established tank. After a week or two (or more), if you see no signs of illness, let them join the rest of your fish.
Wash all plants thoroughly before introducing them in your aquarium. Even the plastic ones.
Make sure your hands are clean before putting them in the tank.
This includes no hand lotions or perfumed soap.
Prevent sudden jumps in temperature. Any more than a 10 degree change in one day can be stressful for fish. Keep this in mind especially when doing water changes, which should be done regularly with dechlorinated water.
Also when changing the water, re-introduce your fish slowly back in to the aquarium. The newer water may have a completely different temperature, pH, and chemical balance than what they were in just a few minutes ago, so don't just drop them in!
Test your water for nitrate, ammonia, and pH levels.
If your water is out of the recommended range for your type of aquarium (see instructions included with each test), then your fish are much more suceptible to illness.
Dry-Tab Master Test Kit
Twenty tests for ammonia, nitrite, nitrate, and pH all in one box. Quick, easy, and accurate.
Tetra Medica Medicated Flake Food
Treats wide range of bacterial and fungal diseases.
Stress Coat & Ammo Lock 2 Starter Kit
Neutralize ammonia and chlorine and repair damaged skin and slime coating with this combo kit.