Probably 80-90% of diseases in captive fish can be prevented by avoiding stress. Stress weakens fishes' immune systems, leading to increased susceptibility to disease. Actually, diseases and pathogens are almost always present in tanks, but a healthy fish's immune system will prevent them from being a problem. Some of the most common stressors for captive fish are:

• Poor water quality: measurable ammonia or nitrites, or very high nitrates.
• The water temperature is fluctuating more than 2 deg F/day
• Incompatible species in the tank.
• Too many fish in the tank (5 adult angelfish in 10g tank).
• The tank is too small for the fish (foot long fish in 10g tank).
• The water is too warm or too cold for the species (goldfish vs. tropicals).
• wrong pH for species (Discus vs. African cichlids)
• pH fluctuations greater than 0.2 units/day.
• Insufficient cover or hiding places present.
• Wrong water hardness for the species (Discus vs. African cichlids).
• Insufficient oxygen in the water.
• Improper fish nutrition (wrong food, foods not varied).

Quarantining new fish is a good habit for all aquaria, but is not absolutely necessary for success. Quarantining is simply keeping a fish in a separate tank for long enough to be certain that it is disease free. Many beginners do fine without a quarantine tank, and object to the cost of another setup. A quarantine tank does cost more, but if a hobbyist has hundreds of dollars invested in fish, it is cheaper to have a separate quarantine tank than to replace fish killed by a newly introduced disease. Also, many of us become attached to fish and do not want to expose our pets to diseases from newcomers, no matter what the cost.

The purpose of quarantining is to avoid introducing new diseases to a stable system, and to be able to better observe new fish for signs of disease. A quarantine tank can also double as a hospital tank for sick fish. Hospital tanks are good because they lower the cost of using medicines and keep diseased fish separate from healthy ones. Quarantine is probably most important for saltwater tanks/reef systems because of the difficulty of treating diseases, or wild-caught freshwater fish because they are probably not disease-free. Quarantining itself can stress fish so be sure quarantine is as stress-free as possible.

To set up a quarantine or hospital tank:

• Keep an extra filter -- a sponge filter is ideal -- or piece of filter floss in an established tank, so that you don't have to keep the quarantine tank set up at all times. Some people choose instead to keep the filter going with guppies or danios (for freshwater) or mollies (for saltwater).
• If you don't keep the tank running, use old tank water to fill the tank. So: old tank water + established filter = instant established tank.
• Add a spare airpump and heater. If you haven't messed with the heater during storage, it should come to wherever you had it last time.
• Consider using Amquel or equivalent when medicating the tank in case the biological filter bacteria are sensitive to the medication. Sick fish are especially susceptible to ammonia. (Note that ammonia which has been bound with Amquel still shows up on a nessler ammonia test. So, if you are planning on testing for ammonia in that tank, you need to use a salicylate ammonia test.)
• For a hospital tank, do small, frequent water changes (even every day).

If possible, quarantine all of your new fish for about three weeks. During that time, gradually acclimate the fish to your tank's parameters: hrdness, pH, salinity, temperature, etc., and watch for and treat any signs of disease.

Do not medicate quarantined fish just in case. Only treat evident, definitely identified diseases. Treating all quarantined fish with a bunch of medicines will just lead to weakened fish and antibiotic resistant bacteria.

Once you are done with the quarantine, if you treated any especially nasty diseases, it is good to disinfect the tank and reestablish the filter. Chlorine bleach or strong saltwater (for freshwater) work well. Be sure all traces of bleach are rinsed off. Another good disinfectant is potassium permanganate (Jungle's Clear Water is one commercial way to get it).

Most important: watch your fish and know what their normal behavior and appearance is. If you don't know what normal is, you can't know what sick is.

Bad signs:

• Clamped fins (fins are held abnormally close to body)
• The fish refuses its usual food for more than 2 days.
• There are visible spots, lesions, or white patches on the fish.
• The fish gasps at the surface of the water.
• The fish floats, sinks, whirls, or swims sideways.
• The fish shimmies (moves from side to side without going forward).
• A normally active fish is still.
• A normally still fish is very active.
• The fish suddenly bloats up, and it's not due to eggs or young.
• The fish is scratching against tank decorations.

I suggest setting up a fish medicine cabinet. It seems like fish always get sick when the store is closed.

• Water quality test kits: pH, ammonia, nitrite, nitrate
• Aquarium salt (NOT table salt. Most table salts contain additives to keep them from clumping. Kosher or rock salt is OK).
• Malachite green/formalin ich remedy
• Methylene blue
• Chlorine bleach for disinfection
• Maybe one antibiotic (Kaynamycin or Furanace)
• Antibiotic-containing food
• Copper remedy for parasites

And for fish big enough to handle:

• Q-tips
• Malachite green or mercurochrome

Fish are gasping at the surface, or very inactive, but there are not visible lesions when it first starts. Their fins may be clamped. Many fish of different species are affected, and possibly the whole tank. If the water has been bad for a while, the fish may have finrot, or streaks of blood in their fins.

• If fish are gasping at the surface, or have purple gills: high ammonia or low dissolved O2 may be the problem; test ammonia, dissolved O2
• If the main symptom is inactivity: test nitrites, pH, dissolved 02, nitrates

Depending on your test results, try the following:

• Ammonia

Change enough of the water to reduce ammonia levels to 1-2 ppm for freshwater or below 1 ppm for saltwater. If that means changing more than a third of the water, be sure the water you add is the same temperature, salinity, hardness and pH of the tank water. It is also okay to do multiple smaller water changes for a few days. Aerate, and make sure pH is at or below 7.0 for freshwater tanks. In addition to or instead of changing water, you can also add a dose of AmQuel to give fish immediate relief. Find out why ammonia is present and correct the problem.

• Nitrites

Change enough of the water to bring nitrites down to below 2 ppm (as with ammonia, if this is a lot of water, match water parameters or do multiple water changes), add 1 tbsp/gallon salt (not all fish may tolerate this much -- start out with 1 tsp), and add supplemental aeration. Find out why the nitrite levels are high and correct the problem.

• Nitrates

Change water and clean the filter. If your filter is dirty, there is more waste material present to break down into nitrate. Start feeding less and changing water more often.

• Low oxygen

Run an airstone. If this helps a lot, the fish probably don't have enough oxygen in the water. Your tank may need cleaning, fewer fish, or additional water movement at the surface from a powerhead, airstone, or filter.

• Improper pH

If pH is too low: make sure carbonate buffering is adequate -- at least 5dKH. In general, adding baking soda at 1 tsp. per 30 gal. raises dKH about 2 degrees. For a 10-20g tank that just needs the pH a little higher, try about a quarter teaspoonful. If that isn't enough, add up to a teaspoonful more. You can scale this up to 1 tsp/30 gal for larger tanks. If the pH is still too low and the KH is at least 5-6 dKH, clean the tank. For long-term buffering in saltwater and alkaline freshwater systems, add crushed coral. If pH is too high, pH down (phosphoric acid) can be added. Don't rely on this stuff, except in extreme situations like ammonia poisoning because it can cause excessive algal growth. To lower pH long-term, filter over peat, or use distilled or deionized water mixed with your tapwater.

Symptoms: Fish look like they have little white salt grains on them and may scratch against objects in the tank.

White spot disease (Ichthyopthirius multifiliis) is caused by aprotozoan with a life cycle that includes a free-living stage. Ich grows on a fish --> it falls off and attaches to gravel or tank glass --> it reproduces to MANY parasites --> these swarmers then attach to other fish. If the swarmers do not find a fish host, they die in about 3 days (depending on the water temperature).

Therefore, to treat it, medicine must be added to the display tank to kill free-living parasites. If fish are removed to quarantine, parasites living in the tank will escape the treatment -- unless ALL fish are removed for about a week in freshwater or three weeks in saltwater systems. In a reef tank, where invertebrates are sensitive to ich medications, removing the fish is the only option. Some people think that ich is probably dormant in most tanks. It is most often triggered by temperature fluctuations.

Remedy: For most fish, use a medication with formalin and malachite green. These are the active ingredients in many ich medications at fish shops. Some products are Kordon's Rid Ich and Aquarium Products' Quick Cure. Just read the label and you may find others. Check for temperature fluctuations in the tank and fix them to avoid recurrences. Note that tetras can be a little sensitive to malachite green, so use it at half the dose.

Use these products as directed (usually a daily dose) until all of the fish are spot-free. Then dose every three days for a total of four more doses. This will kill any free-swimming parasites as they hatch out of cysts.

Another remedy is to raise the tank temperature to about 90 deg F and add 1 tsp/gallon salt to the water. Not all fish tolerate this.

Finally, one can treat ich with a transfer method. Fish are moved daily into a different tank with clean, conditioned, warmed water. Parasites that came off of the fish are left behind in the tank. After moving the fish daily for a week, the fish (presumably cured) can be put back into the main tank. The disadvantage of this method is that it stresses both fish and fishkeeper.

Fishes' fins turn whitish and die back. Fin rot often follows damage or injury. It can also be caused by poor water quality.

Remedy: First, fix the water and remove any fin-nipping fish. Change some water (25% is good) and add 1 tsp/gallon salt to promote healing. If bad water quality or an aggressive tankmate was the problem, that should be adequate. Healing will begin within a couple of days.

If it worsens, decide first whether it's fungal or bacterial. Fungal finrot looks like clumps of cotton on the fins and usually follows injury. It is commonly seen in African cichlids or fish that have injured themselves against decorations. Bacterial finrot is whitish, but not cottony (unless it's columnaris), and can be contagious. The fish then need to be removed from the tank and medicated.

Fungus: For fish large enough to handle, catch the fish, and dab malachite green directly on the fungus with a Q-tip. This is extremely effective. Repeat treatments may be necessary.

For small fish, a commercial fungicide such as Maroxy may work. For severe infestations, try a bath in methylene blue (enough so you can barely see the fish) until the fungus turns blue or for 20 min. If you add methylene blue directly to a tank, you will kill plants and trash your biological filter.

Bacterial: Antibiotic treatment in a quarantine tank. This is stressful for the fish, and doesn't always work, so be sure of what you are doing before you attempt it. If the fish is still eating, the best bet is an antibiotic food. Tetra makes one that works well -- just buy the one for bacterial diseases and follow the directions on the can.

If the fish is not eating, a bath treatment is necessary. A combination of Kaynamycin and Furanace usually works, especially for Columnaris. Again, treat in a separate tank and aerate heavily.