How about the DOS, built by Neptune. I use one for water changes and I have a pair of BRS 1.1 ml/min to handle alk and ca. Any pump without a controller will work with the Apex, just plug it in to the EB and turn it on/off. Real cheap DIY from Ebay, relatively inexpensive dosers from BRS, expensive DOS from Neptune. What do you want to spend and what do you want to get.
How much control from the Apex do you want? A doser that has its own controller to setup the timing and amounts to dose plugged into the EB will allow the Apex to shut it down if a problem was detected. I already spent the money on the controller, the Apex, so dumb dosers from BRS that just plug in to the EB are fine for me.