Don't beat yourself over it lol. We all had to learn from the start. Every new knowledge was picked up at some point. It's been said that the water come with the fish should be completely discarded by the time you set the fish free in the tank. The store water not only can be having different chemical makeups, but it might also carry unwanted diseases and parasites. It is also a good idea to have the new fish stay in a quarantine tank for a few weeks before moving them to the main tank.
There is no such thing as over-filtering lol. Except for the water current, of course.
If the combined filtration (of the three filters) rated by the manufacturer is over 75 gallon, then it should be enough as long as you do not too heavily stock your fish tank or overfeed them. The one way to find out is to test your water for ammonia and nitrite. As long as they stay at 0ppm, you are fine.
If you need more filtration,
Sponge filters are the cheapest, and they don't have much of a water current. The downside is they are not very good looking and they take up space inside the fish tank.
Canister filters are the most efficient, quiet, and not taking up space in the tank, but they are expensive. If you decide to go for canister filter, I would recommend EHEIM. Check out our articles under the fish section for more information.
