Hey Chris!
One of the big issues will be that the pH of seawater in the tank will be min. of 8.0-ish, and the presence of buffer capacity in the tank water will most likely cause precipitation in the line for your kalk, or at best allow for very poor solubility for delivery. This is not a big issue with peristaltic pumps, even if it forms a slurry in the tube, but it will become somehat of a complication long term as it cakes up ocfcasionally in your tubing. FW pH is most often in the 6.8 to 7.2 range coming from most Ro/DI columns, and this will be somewhat acidic compared to the pH of the tank water, enough much more so to keep kalkwasser as a solution until it hits the tank water at your addition point, where dissolution overcomes solubility issues.
Why not use two float switches and a pH controller and use the system as an ATO with safegards: a floatswitch to cut off the pump when the sump gets too deep, one to cut it on as with normal ATO function as the sump level drops, and a kill switch with the pH controller with your probe in the sump to kill th peri pump if pH rises above 8.5. This will give you multiple safegards to stop kalk delivery while avoiding all these other issues when trying to deliver enough kalk to impact your hermatypic needs, yet still not dilute out your salinity. In order to get enough benefit from the kalk, you will find that dilution of your slinity will beome a BIG problem if your tank needs significant calcium and alkalinity from kalkwasser, as the mere volume of the kalk needed, even as a slurry, will cause significant fluctuations in your salinity if you use it as a constant delivery via the pump while trying to maintain salinity based on system volume (the sump depth). You wil find that it will become necessary to increase evaoration from your system with fans in order to deliver enough kalk to keep your Ca and akalinity up and your sainity in check if you have a decent population of hermatypic creatures in the system.
Sorry to take so long to respond, HTH.
I'll be coming down to see Tony soon, maybe we can get together and talk about it.
