Stuck Fermentation - Mash pH? Unhealthy Yeast?

I brewed a Black IPA O.G. 1.067 on Monday (4 days ago). The first evidence of Krausen forming was about 20 hours after pitching. The fermentation soon after got up to a nice active level in terms of bubble frequency, however I was always concerned because the Krausen never grew to the levels I am used to seeing. At most there was only about 3/4" of Krausen during peak fermentation. Also, approximately 42 hours into fermentation, I came home to find that the temperature had risen to 79F. I quickly transfered the carboy to a swamp cooler and got the temperature to 68F after a couple of hours.

Concerned with the health of the fermentation and the noticeable drop in activity at day 3, I took a gravity reading of 1.031. Today (day 4), I read it again and it is still at 1.031. It appears stalled at this range. I have 2 variables that were introduced for this brew that I have never dealt with before.

1. Being my first dark beer on my new all-grain system, I tried to adjust the mash pH using baking soda. I have very neutral water here (pH ~5.7) so I used John Palmer's chart for determining pH adjustments. Using the figure below, I determined I needed 270 ppm HCO3. I didn't do my math correctly and only added 3/4 tsp of baking soda (~115 ppm HCO3 in 5.5 gallons of strike water). I also have very low sodium content in my water so I was not worried about adding too much sodium from the baking soda. Could this stuck fermentation be a result of my mash pH being too low?



2. More likely is the fact that I reused two different types of yeast for this brew. I had about 5 oz of California Common that was pulled from a primary cake and refrigerated for 2.5 weeks as well as about 5 oz of a house ale yeast from a local brewery that was also pulled from a primary cake and refrigerated for 1 day. They were brought to room temperature and pitched with no starter into well-aerated wort.

Thanks.

Too early to tell to be honest being only day 4 or so of primary fermentation. Wait until say day 10-13 and if it is still up in that range, seek alternatives.

What was the mash temp? Is your themometer accurate?

I'd say mash pH is less of a concern.

EDIT: Also the shock of dropping temp might have caused yeast to fall out of suspension a bit. You could always gently rock the carboy to help stir some yeasties from the bottom.

Mash temp was 151.5 at the top of the grain bed (probe is 6 inches long). Until just now I had never tested the accuracy of my thermometer. I just boiled water and my thermometer was reading 210 (I'm at sea level so no adjustments are needed). So judging by this, I guess I have to add 2 to each degree in the future.

I'm not so sure the temperature shock had an effect because I have a satellite fermentation going also, and it too is stalled at 1.031. The satellite fermentor had no temperature control so it likely hit 79 degrees also (and stayed there).

Could I have over-pitched? Too much yeast?

Over pitching would cause it to stop short, at least I wouldn't think so... If anything a lot of what you pitched might have needed some help by way of a starter. Try rousing the yeast be rocking the carboy.