Q: Why is my fish sick and how do I prevent more illness?

A: 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).
Keeping your tank free of disease

Q: Do I need a quarantine tank for new fish?

A: 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: hardness, 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