| Do you already have the Odyssea? If not, I'd suggest holding out for a more reputable brand if you can afford it, or try to find a used one. I saved a ton buying the Nova Extreme Pro used. At any rate, a quick Google search on Odyssea products yields few positive results... generally with aquarium equipment, you get what you pay for. Also, (and check to see if I'm correct here) I've been told that you must buy Odyssea brand bulbs for their T5 fixtures, as they made the bulbs just a little different to work only with their fixtures. That could be a big issue if you're in a pinch and need to pick something up locally.
As for corals, a 4 bulb T5 light over a 20 gallon is a good bit of light. You'll have plenty for LPS, and enough for some easier SPS. I think you could get away with a clam as well; a 20H is only 16" tall so light penetration is less of an issue than in many tanks. | 
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