Enough light?
 

I am setting up a new 90 gallon tank and wondering if my old Tek T-5 36" (6 bulb) will provide enough light for it? The tank is 48"x18"x24" and the fixture is only 36". I will be keeping fish, softies, LPS and a couple of Derasa clams in the 90.

How deep is the 90? I would think fine for the LPS/softies, but probibally not/borderline for clams

The 90 is 24" deep and the old 46 is 20" deep. The clams seem to be doing good in the 46 so I guess I could try to keep them at the same distance from the light in the new tank.

The derasa should be OK with good bulbs. Absolutely not if you're thinking of maxima or crocea.

Personally, I would go with a 4', 8 bulb tek. Would open up more possibilities for livestock in the future. We all say we will stick to LPS and softies but at some point everyone seems to dabble in SPS.