is not needed. just a drain and nuit. change out every 2-3 weeks works for me.I just top off with plain h2o three times and once with a CaMg during a week. I use GH 3 part,and General Organics CaMg+, which is carbonates not nitrates.does a better job of Ph stabilization and doesn't add more Nitrogen.
CaMg cause the Ph to initially drop but within hours its headed back up ands needs adjusted down a little,but don't get in a hurry.

All this is in the flower cycle,as the grow is totally automated and gets kept at an EC of 0.65 or 600ppm

I know BIG MIKE has the best hype on his nuits. but they are way over priced.

hope this makes sence
Daddy