Hi,
I have first hand experience with the out of control algae and "swamp" water. My entire tank ended up turning neon green. I was told the same thing, it's either the water that's being used or direct sunlight is hitting the tank at a certain time of day. Even small durations of sunlight will be enough to make this algae become out of control.
Here's what helped me in the past. I ran a filter (H.O.T. Magnum) for a couple of days and that cleared it up. Along with that, I reduced my lights (12 hours to 9 or 10) and this helped tremendously. Also you might want to check your phosphate levels and add a reducer if they high. But, from my experience these were temporary cures for the underlying problem.
Unfortunately my tank got really out of control last week and my filter broke!! I tried reducing my lights, etc. and it just kept getting worse. I ended up losing my fish as well. So, I just went through the process of getting RO water from the local fish store. I drained the tank, and re-filled it using this water. So far, so good... no more green. We'll see if it returns, but it shouldn't. Sometimes (depending on how hard your tap water is) the tap water filter doesn't remove everything it should.
