Surface area of the substrate is the important thing here, so look for a shallow rather than high tank. For example, in 20 gallons there is the basic "high" 20g that is 24 inches in length, 12 inches width, and 15/16 inches in height. By contrast the 20g long is 30 inches in length, and not so high (not sure the exact figure). So the 20g long would take up more surface space, something to keep in mind when deciding where to put it. Most sized tanks will come in various heights. Of course, the height generally is not important, and the higher the tank the more water/fish. You want the maximum surface area.