I have a 75 gallon tank with an Emperor 400 filter (filters are changed monthly). The tank is lit with a single standard fluorescent tube.I have recently started to have a problem with green water. In the tank are:

9 platies

2 bristlenose plecos

8 zebra danios

8 serpae tetras

Java moss

To be honest, this really doesn't seen like an overcrowded tank to me, given its size.The green water problem started after I added the last group of fish (the serpae tetras).I used to change 10 gallons a week; for the last two weeks I've been changing 15 gallons. I add only the chlorine/chloramine remover and a bit of aquarium salt, no other chemicals to the tank, with water changes. I had green water once before, when I was using chemicals to try to lower the pH to 7.0. As soon as I stopped using them the water cleared.

I've tried darkening the tank with a tarp for 48 hours - didn't make much difference. I'm feeding once a day, only as much as they will eat in a few minutes. I am a bit reluctant to do a massive water change, as the pH coming out of my tap is really pretty high (8.6+, reads at the top of the scale of my pH paper). The tank itself has stabilized at pH 7.6; I think the water is actually soft, but the water company brings the pH up so it doesn't leach the pipes. (They say they add sodium carbonate to increase pH to about 9.1 and alkalinity to 40 mg/l.)My ammonia and nitrite are zero.

Having said that, there have been a couple of fish in the last few months that have died and I haven't found their remains. (Black mollies turned out to be a poor recommendation by my LFS). So I'm wondering if their little corpses are upping the nutrients, although the problem seemed to start when I added the last group of fish two months ago (the serpae tetras).

So, what should I do now? Continue changing 15 gallons a week and hope it clears up? Try a large (50-75%) water change? Thanks for any advice!

Frances