[sphelps]

Sounds to me like your durso is setup wrong. The hole in the cap is too small or your return flow rate is too high for that standpipe. That would explain the issue as the durso is essentially running at full siphon or very close to it meaning it's sucking in a lot of air through the vent (hole in cap) then with the Prodibio added it created a more foaming effect in the drain pipe resulting in the slight back up you observed (same reason why you turn your skimmer off).

That's my guess anyway. You should try a larger hole in your standpipe cap or just remove the cap but chances are once you enlarge it or remove it you'll see it can't handle the same amount of flow, meaning it was previously overloaded. A typical durso setup should never run anywhere near full siphon unless you have a back up.