I’m a bit confused by your post. You have a 42g cube and do ~10gal water changes weekly, but then say your Alk can drop from 9 to 6 over a couple of weeks. Do you not check it in between when you’re doing your water change? How are you keeping it up now? Are you dosing at all or just depending on your water changes to keep your parameters stable?
The first thing you need to decide is how you’re going to replenish your Ca & Alk. There are 4 basic ways:
- Water changes - This is the easiest but most limited in the amount of Ca/Alk that can be added. You can try getting a ‘reef’ salt mix that has higher Ca and Alk concentrations but it still may not be adequate.
- Add Kalk to your ATO - next in terms of ease. The amount of replacement depends on how much you top off which is dependent on how much evaporation occurs. Kalk can also be hard on pumps but a lot of people use this very successfully.
- Parameter dosing with separate CaCl and Alk solutions - you can do this by hand, or automated with dosing pumps.
- Via a calcium reactor - this would be way more than you need.
From the sounds of things, number one isn’t cutting it, so you either need number 2 or number 3.
Now, more to your question, You’re asking about totally different devices - a controller and an automated tester. The tester does just that; it tests your water so you don’t have to. It can’t do anything else. If your problem is that you never get around to testing the water and then when you do you realize it’s dropped way too low then this may be the solution, but I’d check on a Hanna Alk tester. I got one and love it; from the time I pull it out of the drawer until the time I’ve put it away It takes me less than 3 minutes to test my Alk.
A controller automates things and gives you the ability to monitor your tank remotely, which is really nice when you travel, but you are very limited in terms of what you can do remotely. If all you need to do is dose Ca & Alk, you can technically get by with a pair of outlet timers and dosing pumps.
The one other thing that is nice with a controller is the ability to monitor pH which can give you a sign that your parameters are out of whack. My Alk reservoir ran out without me realizing it and I found out because my pH dropped and my Apex sent me an alert.