Forgot to mention: This type of overflow is completely removable. I haven't had to yet thankfully, but if I need to take it out for some reason I just need to drain the water below the bulkhead line, take off the bulkheads, and the box can lift right out the top for cleaning, repair, etc. I've had to net a couple fish out of there in the last year, but aside from some required hand / eye coordination it's not too difficult.