Andrew,
I had been dealing with the exact same issue and after a ton of trial and error I think I narrowed it down to excessive use of GAC (Rox 8.0) to be exact. You dont mention which you use, but since you got it from BRS, I would expect this to be the case.
What I have done is run a small amount (1/3 cup) one day a week for a 220g tank. This helps with any residual organics building up but does not strip the water.
Additionally, alk swings may also be some thing to look at. Check it daily at the same time for 4 days straight between your water changes. You may be surprised.
HTH,
I had been dealing with the exact same issue and after a ton of trial and error I think I narrowed it down to excessive use of GAC (Rox 8.0) to be exact. You dont mention which you use, but since you got it from BRS, I would expect this to be the case.
What I have done is run a small amount (1/3 cup) one day a week for a 220g tank. This helps with any residual organics building up but does not strip the water.
Additionally, alk swings may also be some thing to look at. Check it daily at the same time for 4 days straight between your water changes. You may be surprised.
HTH,