The one thing I've learned most is that SO OFTEN people refuse to listen to advice. "Oh well I've been keeping a tank with no water changes and it does fine." Then when the tank crashes they refuse to believe it's because they wouldn't clean it. Or the people who clean the ENTIRE tank out EVERYTIME and then don't understand why they can't keep fish alive for more than a few weeks.
Here's what I've learned and you can take it or leave it.
FRESHWATER TANKS ONLY
*Aquariums need to CYCLE.( see cycle info here http://www.whichfishtank.com/FishAquariumCycle.html) It takes anywhere from 6-10 weeks for a tank to cycle depending. You can't add all your fish as soon as you set the aquarium up. You have to CYCLE the tank FIRST. Also, CHLORINE must be removed from water every time water is added.
* If you take all the water out of your tank and refill it you RESTART the cycle. 25-30% at a time is recommended unless there are serious problems. My rule of thumb: I change 1/3 of my tanks. 10g tanks I change once a week, 20g tanks every two weeks, 30 tanks every three weeks so forth. I add AQUARIUM salt to my tanks with each change.
* There is usually a BIO FILTER in your filter. Some have a sponge like part in front of the replaceable filters or a wheel. The bacterial colony lives in that and you should NEVER clean that with chlorine water. IF it needs to be cleared up a bit use water from the tank to clean it.
* Take care in adding things to your tank. When adding new fish quarantine them in a separate tank first. Do NOT add water from other tanks to your tank. Pour the bagged fish out over a net into a bowl so that the fish is in the net and the fish store water is in the bowl NOT your tank.
Quarantine any new live plants as well.
Adding too many fish to your aquarium at once will cause a spike in ammonia/waste. It is recommended to add fish in small amounts over time. Rule of thumb they say is 1 inch of fish per gallon but some fish create more waste than others. Goldfish should have a MINIMUM of 5 g per fish. Preferably 10g per goldfish. Also take into consideration the ADULT size of the fish. Many fish you see in the store are juveniles and have yet to reach their full size. So that cute little one inch fish could grow to be over a foot long! Research a fish before adding it to your tank. I had one small fish ( less than two inches) that could kill fish 3 times its size.
I'm sure there's plenty more I could add but at this time I seem to have run dry on ideas :) More later!