I have a diastolic doser that I used for dosing a and b before I got a reactor... Bought it from Natureef.. It works great it needs a timer though $50.. I paid $100 4 month's ago..
I think they are somewhat the same all across the board some have built in timers, some don't usually the cheaper ones... But the problem they all have is clog issue... But if you keep any of them out of the water you should be okay for a while... If you change the tubing to a bigger size you won't have to worry about clogging for a long time... But all micro bore tubing eventually clogs.... All a part of good husbandry...