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You shouldn't need the valve when you turn the pump off. If you do you don't have your return in the tank high enough. I wouldn't put the valve behind the tank if you won't be able to grab it with both hands. A valve that hasn't been turned for a while can need a lot of force to turn. I'd put it...
I really like the Catalina goby. But if you do some research you'll find they won't last long in a reef tank. They need cooler water because of were there from. But like others said there are lots of fish to pick. I'd go with a clown.