to the best of my understanding, it's a separate tank below the display tank that has a gravity fed hose going down to it, and a pump powered hose going back up to the display tank, and it is for putting filters, water heating equipment thermometers, and testing equipment in it. it is connected to the main tank by holes drilled in the tank, and is sealed with something (i'd like to know what this is sealed with). is that basically correct? can you use a basic 20 gallon fish tank for your sump? if i buy heaters for my 50 gallon tank and a 20 gallon sump, i'd need a 70 gallon heater, right? thanks.