Glass top all the way, even my sump is covered. Way less evaporation, nothing can jump and nothing can fall in. With adequate water movement gas exchange is not an issue. As for heat, well the house has central air so no issue there. Another plus is the lights don't get any salt creep or water spots on them, so the lights last longer and with the lights staying in brand new condition it makes up for the light that the glass blocks.
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