what's causing the green water?
 

my friend's dad's tank has green water. I checked salinity, ammonia, nitrate and PH and they are all fine, but his water is completely green. he has a 55 gal tank with a cpr bakpak skimmer, a DSB and some LR. what would cause this green water???

algae feeds on phosphates in the water........tap water is full of phosphates so make sure u dont do any water changes using tap water.....a UV sterilizer is a great usually permanant solution for green water, it kills any free flowing algae (the green), as well as free flowing parasites....good investment

UV will reduce or remove the green water, and I would test your tap water if the green is persistent.
I also think it's most important to look at the cause, Is it the water? Photoperiod (how long are the lights on), Is the tank new? Does he have the right lights or are they $2 homedepot cool whites! Is he overfeeding? Is it getting allot of direct/indirect sunlight? Any of these, or a combination could be the cause. Has he made any recent changes? Through process of elimination you may solve the root of the problem.
If you do use a UV, monitor your fish as you may need to add temporary airstones or do additional water changes as the possible initial mass die off of the algae could cause low oxygen, killing your fish.

Had a green tank last year, used a UV filter, phosphate remover and Clams to consume some of the green. High phosphates seemed to fuel the outbreak.

It finally cleared up after 6 weeks or so, but I struggled alone for awhile. If I had posted quicker I would have saved some grief.

dont do any water changes!!!.......u would be literally feeding the algae and making it worse.......i dont understand what it would have to do with the oxygen levels....some people, like me, keep the sterilizer running all the time and never have any problems with the oxygen levels, as long as there is breakage at the suface of the water to create the oxygen exchange, there aint any problems concerning that....lighting is also a factor in green water after the phosphate level........its good to cover ur tank and leave the lights off when using a chemical to treat the water, but with a uv, i dont think thats necessary because there so effective.

Check you Carbon. Is it for saltwater or freshwater use. If you don't use carbon. Follow the above advice. DO not use the freshwater ones on saltwater. Can cause green water.

carbon is carbon, i dont think there is a difference between fresh and saltwater carbon.......

I believe, if memory serves, there was a post on this board a while ago about someone who had green tank for months and was about to give up when he checked and saw that he was using freshwater carbon. He changed it and everything cleared up almost immediately.

maybe she did somethin else, but decided it was the carbon that made the difference, i mean,i guess i could be wrong, but it just doesnt make any sense to me, and ive been around a long time and never heard of anything like that

u would usually have to change carbon every 2 months, so if she had the tank for months with green water, changin it wouldnt of made a difference cuz the carbon would have been chemically drained