Most of us run our tanks uncovered with respect to having something directly over the waters surface. As your seeing when you use high outputlights the heat generated can really make a difference with covers or with out covers. As Steve mentioned if your MH
canopy has a cover that you not remove this. This cover can serve two purposes. First many have a UV
filtering element to cut down on the amount of UV
light emitted by the bulbs and the second is it keeps water spray from hitting the hot bulb causing the bulb to blow.
Other ways to increase cooling is to increase the water movement. If you can direct a PH
to cause a good ripple at the water surface this will aid in gas exchange and thus increase evaporation therefor lowering water temp. If your running a sump or you can put a fan over your tank that blows are directly at the surface of the water. This will also increase evaporation cooling the water.
Be prepaired for upward to 1 1-1/2 gal
of water evaporation a day when you do these things but your water temperature should no longer shoot up when the lights are on.
What is your normal operating temperature and how high are the bulbs pushing this up?