You could convert the 20G to a refugium or sump w/ refugium - check out this site:
http://www.melevsreef.com
under the DIY section there's a whole bunch of sump designs that can get you thinking. One word of advice - go as big with the sump as you can - will make your life easier - I used a 15G tank for a sump on my 72G tank and it wasn't enough volume. If I were you I would think about going up to 30G for the sump or even more.
Every tank has some algae - you'll never be able to eliminate all of it. That being said - sounds like you have some diatom (brown) and cyano (red) issues. Excess algae is most likely related to excess nutrients (although your bulbs could be contributing if they're old and their spectrum has shifted) - what are your nitrate/phosphate levels? Also, the diatoms (brown stuff) need silicate - are you using RO/DI water? If not, then this is probably where they're getting the silicate - from the water you're putting in the tank. Also - I would get rid of the canister filter - IMHO you don't really need mechanical filtration on a reef tank, and canisters turn into nitrate factorys if you don't clean the filter pads frequently. You're better off just going with a skimmer and a sump/refugium with macroalgae. You also should probably upgrade your skimmer when you go up to 75G - the sea clone 100 is rated for tanks up to 100G - in my experience, the ratings for most skimmers are at least twice the size of the tank they're actually good for. My advice - use RO/DI water (if you're not already), upgrade your skimmer, definitely go with a refugium w/ macroalgae for nitrate removal, get rid of the canister, make sure your cleaner crew is sufficient (I had about 70-80 snails/hermits in my 72G) and that it includes some sand-sifters (nassarius are great for this), make sure you have lots of ambient flow in your tank (I think 20x is good to shoot for) and that there aren't too many "dead spots" where the flow isn't hitting, and go with a deep sand bed in the new tank (at least 4"). Just my 2 cents.
On lighting - if money is a concern then you might want to look into T-5's - they're less expensive than most MH fixtures and less expensive to operate. However - your light is one of your most important purchases and it will go a long way towards determining both how well your tank thrives and the satisfaction you'll get out of it - you can't beat the way MH makes your tank look - I strongly suggest you look into a MH/PCF combo fixture (w/ independent ballasts for the MH and fluorescents so you can do the "dawn to dusk" thing) - its a big initial investment but well worth it.
Hope some of this helps - good luck!!
