Aquarium Disease Prevention;
I have kept up many aquariums (marine and freshwater) during my years of aquarium maintenance. It was more of a challenge than my personal aquariums as many of customers overfed or did not tell me fish were sick until it was too late. That is why prevention is the best remedy.

[1] Cleanliness; regular quality water changes are extremely important. By quality I mean to not over clean the water by taking fish out and washing the gravel. You want to use a gravel vacuum and do partial water changes that disrupt the fish as little as possible.
The purpose of this is to remove organic debris before it can fully go through the Nitrogen cycle, eventually increasing your Nitrates and lowering your ph.
You also want to de-chlorinate the water so as to not stress out the fish or environment. There are many good products for this: Novaqua, Start Right, AAP Prime, AAP Res-Q, just to name a few.

[2] Good aquarium filtration. I recommend two filters for redundancy, and I never totally throw out all media, rather I rinse part of the filter media in used aquarium water so as to preserve beneficial (aerobic) bacteria for proper biological filtration (ammonia and nitrite removal).
Sponge Pre filters are a great addition to HOB filters in particular, they improve bio filtration, especially during filter media (cartridge) changes. Also basic Sponge Filters can really expand aquarium bio capacity and filter redundancy.

[3] Use ultra violet sterilization. UV sterilizers prevent many bacterial, fungal, and protozoa diseases. In addition they help with oxidation properties (Redox Potential ) of the water and in so doing, water clarity.
As well make sure to change your UV Bulb every 6 months for maximum efficiency.

For more information about UV Sterilization, please follow his link: Why use UV Sterilization and how it works

[4] Do not overfeed! Use quality, aquatic based foods, not foods high in cereal, beef proteins and fats, and soy proteins. Some good brands: HBH, Ocean Nutrition, Blue Lagoon, Sanyu, Hikari, Spirulina 20, Omega.
Some brands to avoid: Tetra, Hartz, many Wardley foods

For more information read my article: Quality Fish Food; What ingredients are needed for proper fish nutrition, growth and health

[5] Watch water chemistry, such PH, ammonia (NH3) (0), nitrites (0), nitrates (below 30 ppm), GH, KH (80 ppm or higher depending upon fish kept), etc. Note that ammonia is more toxic at a higher ph! (at a pH under 6.4 ammonia converts to less toxic NH4, which test kits generally do not differentiate).
A constantly replenished GH above 100 ppm provides the needed calcium for proper osmotic processes and healing from infections and wounds. Another consideration related to kH are electrolytes, it is important for ALL fish to have electrolytes in the water column). For more information about kH, calcium and electrolytes, please see this article:
CALCIUM, ELECTROLYTES, MAGNESIUM, AND KH IN YOUR AQUARIUM.
High ammonia and nitrite levels make fish extremely susceptible to infection and will eventually kill the fish outright.
Prolonged nitrate levels above 80 ppm will stunt fish growth and lower fish immunity.


[6] When you do treat for disease, do not over medicate or under medicate, then change water.
MAKE SURE ALL WATER PARAMETERS ARE WHERE THEY SHOULD BE BEFORE MEDICATING!
Follow all the above steps first before adding medication. Medicated wonder shells work well for Ich and are buffered, which makes one of the active ingredients; malachite green, safer (malachite green is more toxic at lower ph). There are many other excellent treatments such as Pimafix (a great Natural remedy for fungus and flexibacter.

See Aquarium Medications; Facts & Information OR for a website with information on many different aquarium medications:
Aquarium-Medications.Com
Including this article: Chloroquine Phosphate, Quinine Hydrochloride-Sulfate for Aquarium

[7] When you purchase fish; First make sure all the fish in the aquarium are healthy (if the fish store has a centralized system [which I do not recommend], check ALL the aquariums). Second, float your fish in the bag for 30-60 minutes for temperature and osmotic stabilization, then open the bag and SLOWLY add your aquarium water to the bag. Third, dispose of the water in the bag to avoid contamination. Fourth, add a shock preventative like NovAqua, Start Right, etc.

Please see this article for much more information about this subject that is updated on a regular basis, unlike this VERY basic post:
Aquarium Disease Prevention

Resource for the only true full professional line of aquarium treatments:
AAP Professional Treatments (Formally Aquatronics)