[shaywood]

My dkh has dropped from 11 to 8.5. Any idea what would cause this, or what I should do to raise it? My CA has remained at 380 (with reactor) and my PH is constant at 8.1 (with Kalk top-off drip daily).

[Ninong]

There are a number of possibilities to explain why your alkalinity suddenly dropped from 11 to 8.5 dKH but before getting into a lengthy back and forth on the various possibilities, it might be helpful if you check out the excellent articles on this topic written by Dr. Randy Holmes-Farley here. He has written several excellent articles on the relationship between calcium and alkalinity and the various ways to correct imbalances.

You say your pH is a steady 8.1. Are you testing it at the same time each day? It would be very unusual for pH to be the same just before the lights come on as it is just before the lights go off. Your lowest readings should be in the morning just before the lights come on and your highest readings should be just before the lights go off. Dripping limewater at night when the lights are off can help mitigate this swing.

Your calcium is a little on the low side at 380 ppm. This is surprising considering that you are using both a calcium reactor and limewater. Adding a small amount of calcium chloride would raise your calcium levels. You might want to consider that until you get the calcium up around 425-450 ppm.

Your alkalinity at 8.5 dKH is fine. Maintaining it at 11 dKH would be perfectly acceptable but alkalinity at 11 dKH is not in balance with calcium at 380 ppm. If you want to keep your alkalinity around 11 dKH, you are going to have to maintain calcium around 475 ppm. My personal preference is for calcium around 450-475 ppm and alkalinity between 10-12 dKH but that's a little higher than what is really necessary. Calcium between 425-450 ppm and alkalinity between 9-11 dKH would be perfectly acceptable and more conventional.