What is a good doser for the undercurrent rdwc

You could set a float valve up to a dosatron which takes from a stock container and mixes it with incoming domestic water line at a set ratio. Most use 100:1

For pH control you could make your own with Arduino or you can buy a bluelab pH doser for less than $400. Not a bad price. Only does up or down. If you need one that does both I'd suggest a different unit.

But usually you only get a pH drop in mid to late flower

Im getting the feeling that a top off rez and the blue lab ph controller would be my best option. Thank you tho.