Planted Tank Guru
Join Date: Jul 2010
Location: British Columbia, Canada
As Patrunkenphat said, a UV sterilizer will get rid of the green water quickly- so that's a good start.
As for the cyanobacteria, Dongo, is a lot of it covering the HC ?
As you may know, cyano usually develops in low/bottom areas of the tank, mainly on substrate, where there is insufficient water circulation, a lack of oxygen, and generally an accumulation of detritus, waste, decomposing plant matter, etc.
It can be effectively removed by spot dosing with H202 (household hydrogen peroxide), where oxygen bubbles will build up on it, and eradicate it, usually overnight, but it may require 2 or more dosings on subsequent days.
It works very well, and if you wish, I'd be glad to provide you with the exact procedure for successfully doing so.
Once it disappears, and in order to prevent it from returning, all you need do is a good clean-up of the bottom levels/substrate/ground cover, and increase/improve water circulation within the tank - this can easily be achieved using a circulation pump, properly positioned, which would also provide oxygenation at the lower levels of your tank. A spray/power bar would also help, as would improved filtration.
Once that is done, you could then concentrate on bringing health back your deteriorating plants.
Hope this helps.