Thought I would chime in as I have been running the 3171 for a year now.
I run no ph probe or controller.
Mine had the same issue of slowing down and it was frustrating.
However I have been running the last 6 months without issues and its very stable.
Alk runs 7-7.3 consistantly.
So what did I do?
I run the effluent at a steady stream and just adjust co2.
I believe if you try and run say 40-60ml, a minute, like I did, it wont hold and keeps dropping.
A just about steady stream on mine is around 120+ mlm.
I dont even measure it any more.
I can look at it and tell if its in range.
I asked Roger if you could run a masterflex, as I have a couple, but he suggested running a higher effluent rate and it works for me.
The higher rate had no effect on tank ph.