Even if your aquarium water looks pristine, it could be harboring a multitude of disease-causing bacteria, fungi, and parasites. Your fish's skin and fins are her first line of defense against these invaders, so they tend to exhibit the first signs of disease.

Consider these common skin and fin abnormalities and their potential causes:

• Bumps on the skin or fins can indicate relatively benign conditions such as parasite infections, lymphocystis, or trauma. But potentially deadly diseases such as Mycobacterium (tuberculosis), Icthyophonus hoferi (sandpaper disease) and Cryptocaryon irritans infection (saltwater ich) can also cause lumps or nodules on or under your fish's skin.
• White spots on the gills and skin are characteristic of ich, a highly contagious disease that can wipe out an entire tank population. If the white spots look like cottony filaments, a secondary fungal infection such as Saprolegnia (cotton-tuft disease) could be the culprit.
• Powdery golden or brownish dust on the skin is a telltale sign that your finny friend is suffering from Amyloodinium or Oodinium pillularis infection (velvet or gold-dust disease). Skin parasites cause this disease, which leads to cell damage, bleeding, respiratory distress, and death.
• Exposed wounds on a fish's skin or fins may be the ugly remnants of a fierce squabble with a tankmate. However, infections such as Mycobacterium piscium, and hole-in-head disease cause similar pitted sores. Skin parasites, such as anchor worms or flukes, may cause the initial skin damage, leaving tissue vulnerable to a secondary infection.
• Frayed and eroded fins and tails may also be battle scars. Long, flowing fins can prove too tantalizing for even the best-behaved tankmates. But bleeding or inflamed edges often indicate a serious bacterial or secondary fungal infection. Bacterial tail rot, for example, is common in some goldfish.
• Excess mucoid slime on a fish's skin means your pet is working overtime to protect her body from infection. External parasite infestations or exposure to irritating substances in the water, such as pollutants or excess ammonia, will trigger slimy skin secretions.

The risk of fatal disease increases any time an infection or parasite infiltrates your fish's skin. When you discover a skin or fin abnormality, treat it as an emergency and investigate the cause promptly.

Your first step is to test the water for ammonia, nitrite, and pH imbalances to rule out environmental causes. Your local aquarium store or fish veterinarian can perform the tests if you don't own a test kit. Also check your heater and air pump to make sure they're working properly.

Next, perform a 25-50 percent water change. Make sure you dechlorinate the fresh water and match its temperature and pH to the tank water. This process will dilute any pollutants or disease-causing organisms in the tank.

If your water analysis doesn't reveal the problem, consult a fish veterinarian or an aquatics specialist right away. These people can l examine your fish, recheck your water parameters, and take a medical history. They may take impression smears of the sores or culture them for bacterial contaminants.

If your fish dies, don't dispose of her body too hastily. You may be flushing away evidence that could save the rest of your fish. A fish veterinarian can perform a necropsy using tissue samples from your deceased fish.

If you can't find a fish veterinarian in your area, consult with your local aquatics specialist at PETCO.

Your aquarium paradise harbors more dangers than you think. But the good news is, skin and fin abnormalities can tip you off to disease before it takes your pet's life.