You can order a tank online like a IM20, IM25 Lagoon, or IM30 and so on which is an all in one and has a filter compartment in the back. They also have the EXT line that is drilled so you can hook it up to a sump.
Waterbox is also popular.
You could also just drill the 40 breeder yourself.
There is about a billion options in the 20g range of varying prices and dimensions. Just pick one you like the dimensions of.
I have a IM25 Lagoon because I wanted a shallow cube because it is a easy dimension to get light coverage and good flow in. Also they have a lot of add-on options for filters and UV or skimmers and so on since IM makes a lot of different products.
