A 75 gallon tank is not too heavy for a floor, but it is important to make sure that the floor can support the weight. The average 75 gallon fish tank weighs about 850 pounds when full. This means that your floor needs to be able to support at least 850 pounds in order to safely hold a 75 gallon fish tank. If you are unsure if your floor can support this much weight, you can always consult with a professional before setting up your tank.