You can make a sump for any size tank and it will always be a plus. While you may not entirely need it since its a small tank, it would do your tank good to have one. It just increases water volume, and the more water, the less fluctuations the water will have, and the better quality the water will be. And when you have good water quality, everything in the tank benefits. I agree with thom fully, and would not use any chemicals ever if it were my tank. Also, your LFS owner may just use that to keep his water in check, considering he has tons of tanks, all connected, which means they all share the same water. He may need it just to keep the stress on the fish down, until the fish is bought, and goes to a bigger, better and more established tank. It could be for alot of reasons.. ones he may not even be telling you about.