I've been having brown algae problems, so at the advice of many on this site, I finally got a handful of nerite snails. (they've been working wonders already, in the scant few hours I've had them) At the store, the sign around the snail tank read "May crawl out of tank....keep the top covered." I have a secure hood, but there are some cutouts in back for my HOB and my heater, with some clearance on either side. How concerned should I be about them escaping to their deaths? There is a good half inch of non-water space at the top of the tank between them and "freedom" (along with about another half-inch of horizontal plastic ridge).