IMHO, with that many plant sites and that small of a res. its gonna happen.

also used GH 3-2-1, 1-2-3 for quite a few grows initially in a 3" tube system with small res. 18 gal/28 sites.

went to buckets with 6" netpots and 20gal/6 plants still had the problem you have,just not as bad

SO, checkout Latewoods legacy a sticky post. it uses a whole different GH reciepe, and it works much better
the Ph runs more like it should. stable until nearing a nuit. change,then it starts changing about 0.1-0.2 units daily

Do you make up with Ph water or a nuit solution?

Read Latewoods legacy
Daddynobucks Reviewed by Daddynobucks on . Low PH problem First time poster, Long time lingerer :p These problems have been occurring for a long time and I have never found a solution so the plants have suffered for some years. :( Right now there are some plants growing in a homemade tube system almost exactly like pictured below. The ingredients consist of PVC fence posts, PVC pipes for the manifold, plastic reservoir, PVC glue to hold the manifold together, Plumbing grade rubber tubing to connect the Rubbermaid top-off reservoir, a pump, Rating: 5