Summarize this article:

Before purchasing your tank inhabitants, it's always best to observe every fish in the store for signs of disease. Even if the fish you want doesn't seem to show signs of ich, it is still exposed to and may carry on ich into your aquarium at home. Some fish have very good immune systems and may just act as carriers to the disease. By introducing a carrier to ich, you are exposing your resident or current tank inhabitants to ich who may or may not have similarly strong immune systems as your new fish. Set up a separate smaller tank so you can observe your new fish for signs of disease. If there are any diseases present, treatment will be much easier but always do the full treatment dosage amounts. Don't think that a small tanks means you only need to do a reduced amount of treatment. When you add new fish to a quarantine tank or any tank, never add the water it was previously in into your aquarium. This reduces the likelihood of transferring tomites into your tank. This prevents the introduction of diseases to other tanks. Similarly, use different sponges and other cleaning tools for each tank. If you can't afford multiple nets, sponges, and cleaning tools, allow for each item to completely dry before using it on another tank. Ich cannot survive in a dry environment. Plants in tanks with fish carry more diseases than those grown and sold separately. Alternatively, you can quarantine tanks for 10 days without fish and treat them with ich treatments to be sure that it isn't infected.

Summary:
Never buy fish from a tank where any fish displays symptoms of white spot disease. Quarantine any new fish for 14 to 21 days. Use separate nets for separate tanks. Buy plants only from tanks without fish.