How do you plan to cool the MH. I have an idea that I am hoping someone will try. iceprobe is trying to use a peltier chip to cool the water in an aquarium will limited success. The reason I believe is that peltiers were designed to move heat from a HIGH heat area to a much LOWER heat area. The difference between water temp and air temp is just not that great (usually no more than 6-12 degrees) The peltier I use to chill O/C'd CPU's is moving heat quite effectively from a 95+ degree CPU heat source to a 74 degree (the larger the difference the more effecient the transfer). From what I understand the temp of the reflector is even higher that 95 degrees possibly topping 100 degrees. Thart would give nearly a 30 degree differential to most rooms.
Here is what I think might work
Using thermal epoxy connect the peltier directly to the outside of the reflector (might be better to use several smaller chips spaced out than 1 large central chip)
again using thermal epoxy connect a heatsink and fan to each peltier.
use an old/cheap computer power supply to provide 250+ watts of metered steady 12V to the peltier and fans. Any left over power could run additional fans, moonlights or any other 5 or 12V gear you might have in tank/canopy/sump
canopy would need to be designed to allow free air flow around heatsink and fan that I assume would sick out the back of the canopy. The better the airflow the better the heat transfer.
my idea was to leave the computer power supply in the computer case and paint the case to match the stand this way it does not look too out of place. I imagine, but have not factored that there would be plenty room inside the case to house ballasts, power strips, UPS, timers, etc to clean up the general area around the tank. Ther should be plenty excess power to run a few case fans to keep all this cool and standard conduit could be used to route power to the canopy and sump to keep things looking neat.
Since I am not setting up the 75G I will not be able to try out this idea right away, but if someone else is willing to give it a try I might be able to come up with some of the parts