Ok, assuming the worst you need to return all of the fish right now and leave the tank running empty for about a week or so. Then start it off with smaller hardy fish (I suggest a school of tetra, 5-7 of them, neons are a good start). Leave the tank running again for another few days before adding no more then 5 inches of fish at a time (meaning 5 1in fish or 2 2.5in fish). Don't follow the inch per gallon rule, that's BS. I suggest looking online for community fish (and I'd like to note that Betta aren't community fish, no matter how lethargic they look in their glass bowels) as several species of Gouramis are aggressive and most cichlids are.
For maintenance, do not clean the physical filter, just change out the carbon cartridge every so often. Don't break down the tank every month, just replace about 1/2 to 1/4 of the water every week. Remember to use conditioner on your tap water to pull out the chlorine (which can kill fish directly or indirectly though bacterial loss).
Test your tank for, at the least, Ammonia, Nitrates, and Nitrites. Ammonia and Nitrates should always be 0ppm, Nitrite should sit at around 20pmm. Once that has been achieved the tank is fish safe and you are unlikely to have any issues with fish disease (unless you overstock the tank).