I use GHL dosers I have 4 units that connect together and dose various chemicals to different aquariums as well as do ATO duties. The GHL dosers can be used separate from a controller if you want them to the SA model has a controller built into it. https://www.aquariumcomputer.com/usa...-white-usacnd/
Here is the older version that is a bit cheaper. These are heavy build German made units that last for ever.