I change the solution every two weeks. Most of the plants are small and the system is closed well to minimize evaporative losses so there is not much loss of water.
HCl is hydrochloric acid. As far as I know, it is ok to use in hydroponic systems.
I know that we can not tell what is in the water by looking at the EC but it gives info on the total amount of ions in the solution. And tap water with an EC of 0.6 has a lot of stuff in it.
Also I know that it contains a lot of sodium, a problem in my area. Sodium is not good for the plants.
My automatic PH adjuster is almost complete. Actually today I'm doing a test run.