Todd,
I would recommend leaving as much as the top uncovered as possible. There are three main forces at work affecting tank temperature: 1)ambient air temperature; 2)direct heating by lights, heaters and in-tank powerheads and 3)cooling effect of evaporation from the surface of the water.
With the relatively small surface area of hex tanks, the more evaporation the better, not only for cooling but gas/oxygen exchange at the surface. Surface water agitation is important to enhance the evaporation/gas exchange.
Try the setup and closely monitor your tank temperature. You might need to put a wall-mount or clip on type fan blowing across the surface if the temp increases too much.
One other consideration: if you have any fish that are prone to jumping, you can get a piece of lighting eggcrate from Lowe's, Home Depot, or other lighting supply house; cut it to shape with a hacksaw, and use that to help keep critters in without restricting airflow.
Dick
[ 06-12-2001: Message edited by: FishDaddy ]
