before you start with peroxide, i would cut the light back to 6-8 hours like said before and do a 10% water change once a week using the RO water until you notice the GHA starting to disappear or die off (then it can be manually removed with a brush). Peroxide does work, but should be a last option IMHO. if the GHA isnt taking over the tank it easily managed by making sure your phosphates and Nitrates are low and keeping up on your water changes. GHA doesn't have an easy over night fix. yes using peroxide will kill it, but it will also kill all the beneficial bacteria and anything else growing on the live rock that the GHA is growing on. managing your phosphates and nitrates, excess food in your tank and your light times, will not only get rid of your GHA problem, but also eliminate all the factors contributing to its growth, preventing it from returning. peroxide will just kill it, but wont prevent it from coming back. if you aren't running phosgaurd or phosban i would start there and cut your lights back to 6-8 hours and keep up on water changes and levels of phosphates. adding more Clean up crew will help, astreia snails will keep the new GHA from growing back, not much will take care of the GHA that is already grown long. if your tank is large enough you could try a sea hare, but once the GHA is gone they tend to die, kind of a waste. good luck, i battled this problem for months, patience and diligence maintaining your levels and water changes is the only long term solution.
