retrofit kits show 46.8" & bulbs show 45.8".....according to reefgeek.com
check them out if you are wanting to do retrofits in a canopy. they have about the best prices around & their kits include your choice of bulbs in the price. they have all the popular brands & very low shipping prices as well
all their retrofit kits:
http://www.reefgeek.com/lighting/T5_Fluorescent/IceCap/SLR_Retrofit_Kits/
for a 75 (per your sig) that is 48x18, a 6-bulb kit would work perfect. you have a few options:
1) get a 6-bulb kit that has (3) standard ballasts
here $449
2) get a 6-bulb kit that has (2) icecap 660 ballasts to overdrive all the bulbs
here $609
3) get a 2-bulb standard ballast kit for the actinics & a 4-bulb icecap 660 kit for the daytime bulbs
here &
here $159 & $359
i personally think option 2 is a bit overkill....no sense over-driving the actinics, so that leaves option 1 or 3. then it depends on what you want to keep......light demanding sps corals, clams on the sandbed, etc then go w/ option 3. otherwise option 1 will be plenty sufficient
keep in mind that over-driving the bulbs will get you more light out of them (probably 80w each instead of 54w) but it will also mean that you will need to cool them directly w/ a fan & also they will be need to be replaced sooner (about every 9-12 months vs 12-18 months).
cooling them is best achieved by having a pc fan blowing front to back of all the bulbs over their "hotspot"...which is the end of the bulb where the writing is at. this will be much better than a fan blowing lengthwise down the bulb & will help keep them cool which = longer bulb life
hope that is all helpful
