Planted Tank Guru
Join Date: Jul 2010
Location: British Columbia, Canada
Do you have a picture of the problem ? Cyanobacteria (blue-green algae) is essentially that color, and I haven't seen it as a purple color - or are you describing something else that you have besides cyanobacteria ?
If it is actually cyanobacteria, you can treat it by dosing H202 (hydrogen peroxide) at close range, using a plastic syringe or test kit pipette. Turn off your filter(s), lower the water level so the algae is readily accessible & dose it. You'll see it being covered in bubbles (which will soon begin to float upwards) - a sign that it's working. Leave the filter off for at least a half hour after dosing. Refill the tank, start the filter, and leave it overnite. You should see the improvement the following day. If there are still traces of the bga, repeat the treatment.
I've used it a couple of times - it works.
Then you'll need to deal with the cause, so that it doesn't return.
If you actually have bga, I'll be glad to fill you in on how to keep it from returning.