do the 10gallon tank sump. i have one on my 20L and it works awesome. i made mine into a refugium, but same idea. it was very easy to cut some acrylic (from home depot) to fit, silicone it in and waa laa! very cheap and compact sump. i have my skimmer hang on it, my heater, auto top off system, some LS, LR and cheatomorpha, 18w actinic/10k light and return pump in it. it gave my system an extra 6.4 gallons. you need to make sure that you have room in it to take on extra water if the power fails and the pump stops. that is why i have a sump/fuge that normally handles 6.4 gallons. the rest is overflow protection.