I did re-introduce biology into the tank. It had a bio-wheel that was cycled in the filter, and lightly rinsed the 2 stages from my Pir. filter that didn't include carbon. 1/2 of the new water was also from the pir tank. Params have been a bit up and down, but not crazy since, and I did get a master test kit to keep a better eye on things.
I also got some Prime. I'll probably use up the Aquasafe in my pir tank as there have been no issues there, and use prime in the pleco tank. I suppose I could also compare to see if one is working better than the other.
As for the deaths... I do believe it was Ammonia. All the guppies started sitting at the top of the tank gulping air despite the 8 inch air stone I have. Their gills were also a bit inflamed, so I don't think they were getting any oxygen... I got about 2 days notice when they moved to the top of the water.
Thank you to all who helped! I'll be a bit more diligent to check all parameters from now on.