The algae is back overnight? Wow. I think at this point we have to figure out whether your water is high in phosphate or your rock/substrate is leaching phosphate. It's either got to be that, or something's up with the filtration.
I might have missed this, but are you on town or tap water? What area are you in?
Can you take some substrate out, put it in, say a litre of fresh water (test the phosphate first), then test again in a couple of days? Do the same with one of the rocks. If something is leaching out of the rock or substrate, if you eliminate that it should get rid of the issue.