i know u said u couldn't change the tank now but how aout this, were the "wier" is at the top corner of the tank would it be possable to put in an extra peice of glass thats say 1" above and 1" below the waterline, that goes across infront of the outflow, so that not so much surface water would be drawn down, the advantage is that u'd unly have to drop the water level by 6" max to add something like this