Welcome to the board, Mossy Oak.
I have extensive experience with many sizes t5 and can assure you, the more the better. I use a 2-tube 24" just as fill-in for a few plants and it's not all that bright, but the set up is in a south window so light really isn't lacking. I normally recommend 48" fixtures since they are the most common and the best value for the money you spend. The 24" fixtures and tubes are disproportionately higher in cost (ie, not 1/2 the cost of 48") but sometimes the space dictates the size.
Now as for your question, 6 tubes would be the minimum I would plan. The 8-tube is only 21" wide and less than 24" long. This would barely cover the area of your tank. Here is a link for 24" fixture:
Buy New Wave T5 Fluorescent Grow Light 2ft 4 Bulb 192W (The link says 4-bulb, but it really is 8-bulb).
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Ross
http://orchids-ross.blogspot.com/
I don't do most Dendros or large, "floofy orchids". For me it's minis (like Angraecoides), Masdies, Paphs, and others that crept in somewhere along the line. See my orchid list for complete collection (usually not current.)
Lighting
Linear Mode
