I've had the same problem with low calc. in my ReefCrystals salt. Although I'm only on my second bucket of this stuff, it's never tested over 400, usually around what you're getting, 375. The last five water changes, I've added 1 tsp. of Kent TurboCalc to each 5 gallon bucket. This yields a calc. reading of 435. With this new procedure and the addition of a Kalk reactor, my tank calc. has been steady in the 430s. So, start off by adding enough calc. product to the main tank to bring it up to where you want it. Then try adding a little calc. product to your new water before adding it. With the help of a calc. reactor or kalk addition, you should be able to keep calc. where you want it. The alkalinity is great in reefcrystals. Consistently about 10 dKH. The other problem I have with crystals is low Mag. levels. Increasing the Mag. levels of your makeup water will also help with your calc. issues.