a 20g tall is 24" so your cheapest T5 HO option is going to be the current usa 24" nova extreme 24wx4 at about $170 just make sure you get the
latest model since the older versions did not have individual reflectors. bulb upgrades will also get the most out of it. if you cant find the latest nova extreme look for the 24" sundial T5 HO 24x4w fixture although the extreme is $50 cheaper and has the more updated reflectors but it comes with built in timers and perhaps looks a little cooler. expect to pay in the $200-300 range for a single 150w MH fixture or pendant. pendants have to be hung from the ceiling or hanging arms but are usually cheaper than fixtures with legs. I'll stick with current usa for suggestions since they are fairly mainstream and easily obtain almost everywhere online. 24" sunpod fixture with 150w halide cost about $275 and sundial 150w halide and 32x2pc pendant cost around $230 and can be had with or without hanging arms (for the same price for the time being). I'm not trying to suggest you ONLY look at current-usa either. any reputable brand 24wx4 T5 HO with individual reflectors will work and anyway you can get a 150w halide over the tank will work just fine. if you have more money to burn there are better higher end light fixtures and pendants to be had.
24" nova extreme T5 $170ish
24" sundial T5 $220ish
sundial MH/pc pendant $230ish
24" sunpod MH $275ish
current usa hanging rods (can be used with sunpod/sundial or any pendant for that matter) $$ dont know but its being offered free with the sundial MH/pc purchase for now