I have been using my city water for that last year and a half without incident. My city water test zero for phosphates as well.
I bet this would be your problem. Why has it taken a over a year to crop up? Dunno... Perhaps a change of food brand or amount suddenly spurred it. Perhaps changing salts or not doing enough water changes did it. Really hard to tell. Its a little odd that you have zero phosphates yet have "increased growth of green hair algae" in your tank. What phosphate test kit are you using and how old is it? Also, have you taken a nitrate reading in your tank recently. A high nitrate level can also cause algae growth.
Anyway, a RO
unit is ideal. I am not going to tell you that you *have* to have one, as quite frankly, some people don't use them. But I think that would solve a lot of your nutrient import problem. Cutting back on feedings may also help.
Wait around to see if anyone else has comments about using O3 for algae. I have never heard of it, but perhaps someone has and the LFS
isn't totally crazy...