If this were my tank here's what I'd do:
1. Start a refugium. This can be done inside the stand, or as an adjacent stand-alone tank. You'll want to light it 24/7 until the macro algae you've placed into it has established itself. One easy way to do the external fuge is to use a powerhead to pump to it, and let water gravity drain back to the display or sump. Make sure the gravity return is BIG so that it won't clog.
2. Start running a canister filter with phosguard or phosSorb, etc... Like Seamaiden said, although your phosphate tests may show that things are "within acceptable range," my experience has been that ANY detectable phosphate will cause problems.
3. If you're not using a skimmer, now might be the time to think about getting one. For this size tank, an aqua-C Remora would do nicely.
4. Stop dosing all supplements except kalkwasser. Start daily top off with kalk drip. This will do many things for you. First, the elevated pH tends to cause nuisance algae to crash. Calcium hydroxide also functions to precipitate out phosphate molecules.
5. Lay off the water changes for a few weeks and see where things settle out.
6. Since you are not keeping any light dependent critters, you can shut the lights off altogether until the algae recedes. You'll want to aggressively skim and use phosguard, etc... while doing this.
7. Try taking the suction strainers off the intakes of your powerheads. They get clogged very easily and the resulting reduction in flow contributes to algae's abilty to cling onto things.
8. The critters you have are not the best algae eaters. I like emerald crabs and snails. Reeftopia.com has the cheapest, best deals around (Tell Edie that Eric in Fairfax sent you). Go with 100 of their really small Trochus or Astrea snails. (it's like $38.00 for a lot size this big) You'll see results almost overnight. Don't get the nudibranchs, they're cool to look at, but pretty much useless.
9. You can also circulate your top-off water thru a phosguard filter before mixing salt. That way you'll take an extra measure of removing silicates and phosphate molecules before the next step, salt.
10. Use only Instant Ocean salt.
Hope this helps. Keep us informed and update with a pic when you can.
Snapper