Neptune apex atk v2. It is a standalone and does not need an apex to run.
Only ATO on the market with 3 sensors and the last one being a mechanical float valve. I would never run a dual optical sensor like the osmo 3.
Selling a Avast K1 that I got from Issac here. He replaced the motor when I got it and I used it for about 4 weeks. Works perfectly
https://www.avastmarine.com/products/kalk-stirrer
$150 firm, not taking anything less for it.