I have a 5 gallon AIO (all-in-one) with the sump in the rear. After searching the entire room for a missing Amano shrimp I gave up...few days later I find my one and only Ghost shrimp has now vanished so I start looking in the rear. Bingo, they crawled up over the back wall so I assume if they really wanted to get back into the display they could if they wanted. With that being said what do you think about leaving a shrimp or two back there in the dark? Granted it does need cleaning there is a lot of surface area for algae to grow so they are probably grazing away. Would you leave them? I can remove them a few times a year and vac the bottom of the sump out. No snails are yet to make their way over there, but I am sure a Nerite would clean the sump out nicely!