If you put your sps corals in the top have of your tank, I would not think you would need to run the T5s more than 10 hours a day... see what others think
Here a another good :read:
http://www.reefsanctuary.com/forums/equipment/50016-basics-lighting-reef-systems.html
partial extract...
T5
T5s are the work horse of the hobby. They are very flexible and have many applications. The best MH systems today come with T5 supplementation, and T5 systems are beginning to hit the market with LED supplementation to produce shimmer. This technology is the most commonly used one today.
T5 means in fluorescent lighting means a straight tubular bulb, with connectors on either end, 5/8” thick. T12 is 12/8” thick and so on. As a side note, PC manufacturers are making claims that their bulbs are T5 PC. This is an outright falsehood in lighting terms, used just for marketing. The T denotes a straight tube, not a curvy one.
T5s are a straight, thin bulb. If MH is all about the ballast, T5 is all about the reflectors. The bulbs being so narrow, it is possible to wrap a reflector around it and focus the light from the bulb directly into the tank. This increases light and penetration as much as 300%.
When choosing a T5 light system, consider only the systems that have individual reflectors for each bulb. The difference in performance like I said is as much as 300%.
Being thin, this also enables us to “stack” more bulbs in a given space, increasing the intensity and amount of light available in the tank. It is very easy to underestimate how much light a quality T5 system can produce.
The stacking of bulbs is really where T5s come into their own. There are many specialized T5 bulbs out there, and you can mix and match them to make the colors on your fish and corals really pop. A UVL 75/25 bulb produces a lot of red. My common firefish glows fluorescent red and orange under this bulb. This gets very complicated, and there are more bulb combinations that work than there are aquarists, and a lot of it comes down to preference.
But some generalities about the bulbs. They come in Purple (Actinic), which are 420 NM (nano meters) in color; Blue (which many are labeled actinic, like Current, but are not) which are 460 NM; and Daylight. Which are around 10k (10,000 degrees Kelvin).
Add into this mixed bulbs, which are a combination of Daylight/Blue, Red/Daylight, and it gets complicated.
As a rule, Daylight bulbs produce more PAR (photosynthetically active radiation, light in the spectra used for photosynthesis, the only accurate measure for the hobby) than the others. Some of the individual combination bulbs counter intuitively produce more PAR than a daylight bulb. And actinic bulbs produce very low PAR. What they do is make things fluoresce.
The mainstay bulb is the ATI Blue Plus. This bulb for whatever reason, is one of the highest PAR bulbs out there. Being a blue bulb, it emits some light in the actinic range, and will induce fluorescing. About half the bulbs in your system should be Blue + bulbs.
Bulbs also come in standard lengths, 24”/24 watts, 36”/39 watts, 48”/54 watts, 60”/80 watts. When we talk T5, we really mean T5HO, because all these bulbs and fixtures are driven for High Output. You can over-drive these bulbs with an Icecap ballast. A 36”/39 watt bulb will become 54 watt; a 48”/54 watt will become 80 watt and so on. This is T5 VHO.
There are also 18” T5 bulbs out there. I really recommend against them. The bulb choices are extremely limited, and they are all cheap Asian manufactured bulbs. None of the good performers come in this size.
Active Cooling is also very important for T5. This means a fan. The cooler the bulb is kept, the more PAR it will emit. For every 2 degrees the bulb reaches over 79 degrees, it loses 1% of its potential output. So, a bulb which reaches 108 degrees, which is common in uncooled fixtures, loses 50% of its output.
For penetration, with a good quality reflector, T5 can do 24” very well. My ATI does 200 PAR 25” deep in my tank, which is better than many MH systems. I think many of the bad experiences come from people who do not use quality reflectors, actively cooled fixtures, or a combination of these. For the tanks most of us have, T5 will work great.
For up to 30”, and ATI or Aquactinics reflector will be just fine, Beyond that, which really is not that common, go MH.
For bulbs, stick with ATI, UVL, or Geisemann. There are individual bulbs out there that may come into play, like the GE 10k daylight or the Koralen Zucht Fiji Purple; but these outperform the other manufacturers for the same price, or just a buck or two more. A Current daylight bulb is around 250 PAR, an ATI daylight is 370 PAR, without reflector.
For fixtures, they commonly come in 4 bulb, 6 bulb, and 8 bulb setups. Aquactinics also makes 5 and 7 bulb setups. 6 bulb setups produce the most PAR. 8 bulbs produce the same par over a wider area
ATI fixtures are of much better quality than everyone else, then Aquactinics, then a number of manufacturers in the pack.
MH versus T5
Don’t let anyone tell you different. For all intents and purposes, MH and T5 are technically equivalent. With very few exceptions, you would not be anymore successful with one of these systems over the other. And those exceptions come down to animals that require expert care, like Ritteri Anemones, and individual animals.
The choice between these two technologies comes down to personal preferences. MH produces shimmer, which is very appealing. T5 doesn’t produce as much heat and can be fun to play with colors. The best coloration in corals I have seen comes from T5; the best growth comes from MH; tastes great, less filling. That’s your call.
Recommendation
If you have no idea what you will be stocking with, or what you like, my recommendation is a 6 bulb T5 system with individual reflectors and fans. I think it is most likely to give you the results you want, and you will avoid having to upgrade later. Want to do a soft coral tank? Run only 4 of the bulbs. Want to move to SPS (Small Polyped Scleractinian)? Use all 6. They come in many price points, sizes, manufacturers, are relatively inexpensive to run, and you probably won’t be kicking yourself in six months and looking to get a new fixture.