40 gallon tanks are divided into 40 high which is a fish tank with the dimensions mentioned above, a 40 long, which is essentially just a long fish tank, and 40 gallon breeder tank. As you probably noticed, the usual 40 gallon tank has a rectangular base, making it a little hard to reach inside.  Breeder tanks are made with a square bottom and have more floor space. That is to make it easier for them to have easier access to their fish. This dimension is better for fish keepers that plan to breed their fish due to the ease of catching fish and handling them. However, breeder tanks aren't exclusive to fish breeders only. There's a bunch of people that appreciate breeder tanks for their form and use them for other pets like turtles or want to create landscapes underwater. There's much space to go crazy with and experiment, and that's why this tank is so popular. Our breeder tank of choice would be the SeaClear Acrylic Aquarium Combo Set which was so good we even awarded it as our best overall tank of choice.