Mine is a 10 gallon. I would not aprove of a 5 thought because you need to construct a buble trap and i dont think that would be posible in a 5 gallon enless you have very low flow rates.(i use a 650 ghp pump)
Y do you need a sump? then you need to buy a overflow and a pump, and pvc(trust me you will end up going to the h-depot more than once you prolly wont get it rite the first time) if all you want to keep is two clowns, and some lr and like one peice of corol than you shouldent even consider your tank a reef tank. a powerfiler would do.
It's not so much about the size of the sump, it's that your intended gear will fit in it. IE, chillers, skimmers, return pump, fuge, UV sterilizer, Nitrate reductor coil, ETC.... So you should put some thought into what is going IN the sump to make sure your sump is large enough.
You wouldn't want to get that really sweet skimmer only to find out it won't fit in the 5g sump. A 10-20g would suffice. A 30 or 40 would offer even more choices.
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