yellow water is bad bad bad! the causes is clear! its from inadaquate filtration, excess feeding, too much fish, lack of maintanace, etc. basically it all points to excessive nutrients. i believe it(the yellow color) has something to do with free floating micro algea. they are feeding on the excessive nutrients . regardless of the cause your plan of attack must be .......first!........ major water change ...then a strict water change schendule must be followed. i would say every 3 weeks at this point and eventually going for every 4 weeks. next attack filtration. increase by changing media more...better yet upgrade equipment to something better. even better stop using canister or hang on. media can leach out and be source of issues. marine tanks only need a good skimmer. with the technology today some skimmers are so efficient thats all you need along with water changes and a sump never ever hurts ( more like makes everything better).