Gwen, the bases get pretty warm on these so the fewer the better. The base is the ballast for the tube. The tubes are designed to run at 95 degrees F which is why they are warmer than florescents. You could actually hang bulbs outside tank and use a fan to dissapate the heat befor it gets into the tank. Here's what I did for mine
The fan is at the top and blows through the light bank exhausting at opposite end. The 4 - 48" t5s run pretty warm but only raise tank temperature about 3-4 degrees. By the end of the 12-14 hour "day" cycle the tank is approx 84-86 degrees F.
__________________
Ross
http://orchids-ross.blogspot.com/
I don't do most Dendros or large, "floofy orchids". For me it's minis (like Angraecoides), Masdies, Paphs, and others that crept in somewhere along the line. See my orchid list for complete collection (usually not current.)