snails...
snails...
ok, so the water you use isn't the problem. you said the fish all died off a month ago? have you done any major water changes since then? a thought just popped into my head that perhaps a fish died that you were unable to remove, which is fueling the algae growth.
another thought: do you have a refugium set up in the sump? if so, you could add macro algae (chaeto to start) that will suck up the excess nutrients and out-compete the nuissance hair algae.
as for the snails, my mexican turbos are pigs. they have kept my tank hair algae free it's entire life (~6 months). the lawnmower blenny gets in the places they can't reach due to their huge size (almost the size of half or silver dollars). another thing to add might be a few hermit crabs. i have a few zebra hermits that never stop moving and eating (one has molted at least twice in the past 3 months!).
worst case scenario--since you don't have any fish in the tank, you could always take the rock out and scrub the algae off with an old toothbrush, but unless you get the phosphate down and the ammonia down, it will continue to come back.
hope this helps.