Alk is has been high (just tested 15.7) for about 3 days did water change but don't seem to be going down using kent salt, calcium reactor is turned off. What should I do
1. Waiting for it to drop (usually best)
2. Adding a mineral acid such as hydrochloric; will cause a big temporary pH drop
3. More water changes.
Do you know how high your demand is? I presume reasonably high since you use a reactor? The higher it is, the better of a choice 1 is since it will drop faster.