Q:

Will Sparkolloid slow or stop fermentation?

Background: My wife drinks cider like a fish drinks water and buying cider at the store was leaving me pretty much broke. The LHBS turned us on to the idea of making our own a couple years ago so we gave it a whirl. Preservative-free apple juice and a Wyeast Cider smack pack yields a decent dry cider that is super easy to make. The best part about it is that she will drink it without much fussing (She is normally pretty fussy about food and drinks). The worst part about it is that it takes a long time to clear. To get a clear product faster, I tried my hand at adding Sparkolloid when there was probably about 10 gravity points left in fermentation. About 8 hours after the addition, it appears that fermentation slowed significantly, nearly stopping. I'm not talking about just the air lock not bubbling as much. The production of CO2 bubbles coming out of the cider itself has nearly halted. 
So - did Sparkolloid have something to do with the slowed fermentation? I'm leaning towards no since I could not find anything supporting this idea on the www, but figured I would ask you experts.
Bonus question - When is an appropriate time to add something like Sparkolloid to wines and ciders? In my beer production, I normally just add Irish Moss near the end of the boil and I've never had a turbid batch.

A:

Everything that clears your drink by dropping particles from suspension will also drop yeast from suspension - and yeast can only work properly when suspended in the liquid. So yes, adding any clearing agent is bound to slow down or even stop fermentation.
To be sure, wait until gravity does not change for 3 consecutive days and only then process with additions like that. Some yeast will always stay behind, so refermenting in bottles will still be possible (if maybe slow).

