Next, I want to measure ammonia and CO2 directly and looking for sensors for these. I am also going to work on a semi-auto water change system, (mostly in view of the larger saltwater tank not the smaller and easier freshwater tank.) Going to put two water barrels in the garage and run plumbing into where the tanks are. Eventually, I will push a button to empty a certain amount of water into a waste barrel, then, push a second button to move a pre conditioned amount of water into the tank from the clean barrel.
The following CO2 sensor (for air) was looked at by mistergreen:
Thread: Affordable CO2 sensor?
It certainly is cheaper than CO2 sensors made specifically for aquatic purposes.
Ammonia probes run about $300 and up, last I checked.