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 dechlorinate the water so as to not stress out the fish or environment. There are many good products for this: Novaqua, Start Right, Stress Coat, just to name a few.

[2] Good 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).

[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.

[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.

[5] Watch water chemistry, such PH, ammonia (0), nitrites (0), nitrates (below 30 ppm), hardness, KH (80 ppm or higher), ECT. Note that ammonia is more toxic at a higher ph!

[6] When you do treat for disease, do not over medicate or under medicate, then change water. 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.

[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, ECT.