I would recommend an even slower drill speed. One of the few places I've been able to find much detail about drilling an aquarium is on
Diamond Drill Bit & Tool's web site. Follow the link above, click on "How to Use Diamond Drills" and read through the whole page. They recommend 500 rpm for a 1" hole and 250 rpm for a 2" hole. I believe this web site has lots of good suggestions including drill speed, drill pressure, cooling, hole location, etc.