There are so many common mistakes in aquariums. I would like to name some of the common ones: 1. Inappropriate fish selection 2. Poor tank maintenance 3. Overfeeding the fishes. One of the most common mistakes that I see from people is that they put the fish species that are not compatible with each other in a single tank. Like putting large fishes together with their small counterparts or putting fishes that are small individuals together with their larger counterparts. You must be aware of the fact that all fishes have their own territories and they swim, hunt, and interact with each other therefore it's always better to have fishes of the same size and swimming habits in a single tank.