Algae
Squegees? no - Keep fish in the water gap. The sort that scavenge algae off glass.
Have some predator fish too, and some fishermen...
(BTW, Gary, any chance of you posting a mini version of your wife's fantastic art work for this scenario. I'm slucky enough to have seen it but the rest of the HZ hasn't. Don't post it too large or it'll get stolen by every wannabe on the net, (C) or not!