As a fish keeper you will in all likely hood make mistakes of some sort building your tank, but avoid making these 5 mistakes:
#1 Not Cycling: This one of the surefire ways to kill your fish. If you don’t establish a good Nitrogen Cycle then ammonia & nitrites will accumulate, killing the fish. The best way for freshwater aquariums is to introduce nitrifying bacteria and add ammonia to begin “fishless” cycling. After the ammonia and nitrites are gone you can begin stocking. Though with saltwater tanks you can just introduce live rock and sand and then just take test until the water is ammonia and nitrite free.
#2 Overstocking: Know what space a fish needs to live, create a plan and stick to it. Often overstocking can overload the Nitrogen Cycle as well as create swimming space problems. Resist the temptation to buy ‘just one more” specimen.
#3 Not Acclimating: Face it your tank water is not like the water that the fish you just bought is in. If you just net the fish and stick it in you are essentially putting it in a completely new environment. Acclimation is the best way to help them adjust.
#4 Improper Stocking: When you buy two fish you must make sure they will live together peacefully. This is one of the many reasons beginner hobbyists leave the hobby. Almost every fishkeeper has made this mistake, only to wake up one day finding one bloated fish fish floating belly up on the water and the rest missing.
#5 Not Doing Your Research: If anything do not make this mistake, incompetence in what you are doing will only lead to failure. In the end not knowing what you are doing will cause you to make the most possible mistakes. The more you know the less mistakes you will make.