For logistical reasons (in other words, I didn't have enough buckets) I replaced all but a few gallons with new water.
I actually don't think it was the water change that was the issue, I think the substrate got too stirred up with all the vibrating during the move (we had to push it across the tile floor, there was no way we could lift it at all).
Since I've got a really deep substrate, there's also a possibility an anaerobic spot was stirred up...
I may never know for sure, but things are finally looking up now. A few days ago the Pristellas were spawning, so that's definitely a good sign.
The new Calico BN plecos from Saganco are also doing really well.
And thanks, James.