Water Chemistry Guru's

[sphelps]

Most likely it's just a simple build up of nitrate that accumulated over the 11 months as a result of over feeding and or lack of nutrient export. Could you post your tank and sump size plus the skimmer you use?

Once a build up of nitrates is present it can be difficult to bring down, 15% water changes biweekly alone will have little effect. First fix the source which could be an over stocked tank, over feeding, or poor skimming. Then do larger water changes less often, for example 30% monthly is more effective than 15% biweekly. However you should consider larger changes in the 50% range if possible to make a larger difference. Even doing this will take some time.