Should be no problem if you spec your T5's right. But if your going to spend all that money on a T5 fixture on the 75g, IMO, I would put the money towards a MH setup with actinics.
A lot of people have been able to keep clams under T5s. You need a quality unit with good parabolic reflectors and possibly overdriven with Ice Cap ballast like the Aquactinic Solar Flare fixture.
I've had clams under T5's for 7 years, no issues. Like Warren said, you should look for a quality fixture that has INDIVIDUAL, PARABOLIC, HIGHLY POLISHED reflectors. There are many cheap fixtures on the market that are priced so low you wonder how they can offer them at that price. The first thing you need to look at with those fixtures is the reflector. Most times they are a flat piece of aluminum above the bulbs. This reflector is NOT what you want if you want clams.
If you plan on kepping SPS I would just go for a Halide/T5 combo. Your gonna spend a decent amount of cash for a really good T5 fix, why not go the whole nine yards and get the halides. Then you should have no problem keeping what ever you want
get a current outer orbit the fixture have every thing in it at least mine do 2+250mh 4+39w t5 and 18 led 9blue 9 white i like the fixture is give u all the lighting u should need to grow any thing
I have a clam and it is under ho 6 t-5s and it is doing fine. The lights area big matter of pref. esp on this site. poeple tend to go with mh because they like them and the other half go with t-5 to save on money and check out the colors. It all depsneds on what you want and what you can afford with a electric bill
get a current outer orbit the fixture have every thing in it at least mine do 2+250mh 4+39w t5 and 18 led 9blue 9 white i like the fixture is give u all the lighting u should need to grow any thing
people are always speaking of the electric bill. it's not much different either way. 6 x 54 watt t-5s or 6 x 39 and two 150 watt halides...it's about the same.
there are some heat considerations however.