SPS and T5HO VS MH

[Stones]

With your 4 bulb T5HO fixture, you shouldn't have any problems keeping SPS corals in the top 1/3 of your aquarium. Any lower than that, you are going to need to get a 6 bulb fixture to help increase PAR levels.

I have a SPS only 90 gallon tank and decided to switch from metal halides to T5's last Christmas. After 7 months of running the T5's, I couldn't be happier. I replaced a 2 x 250 watt HQI metal halide setup (which I had used for the past 4 years) with a 8 x 54 watt T5 fixture that does not overdrive the bulbs. Colours are improving in my acros weekly and I don't have problems growing acros on the sandbed at a depth of 24".

Like the others have said though, good individual reflectors and active cooling really help boost the PAR output of T5 fixtures so that you can keep corals you would normally need metal halide for.