what are the dimensions of this tank? 36x 20 x 20? it's important to know how deep the tank is top to bottom, and how long it is. most halide fixtures that come in 36" have only 1 halide lamp. i would not suggest this for your tank. the outer edges will be barely lit.
i probably wouldn't go with 4 x 39 watt t-5 ho
either. i would want 6 lamps. look for a 6 x 39 watt t-5 HO
, that is individually reflected. you don't want a single piece of flat aluminum behind all the bulbs...you definitely want it to contour around each individual lamp. this is very important. it increases the potency of the lamps tremendously.
also, watch out for those ebay no-name fixtures. many of them are not UL listed and have poor construction. in all cases you'll need to put better bulbs in them and in many cases, you'll need to change the ballasts. they can and have started house fires as well.
yeah, that plastic in the center is structural. i wouldn't remove it.
if you are price shopping, you are better served looking for a used quality
fixture on the forums, and craigslist...and don't even think about LEDs! they are still, quite expensive.