Jeez, sorry to hear about the loss.
To me at least, nothing jumps out as the obvious culprit. It could be any one thing (or any combination of things) and it would all be based on excessive speculation.
Obviously I don't want to suggest something I don't know for sure, but I feel like when I've bought things off Amazon in the past I've had items that were returned sent back out to me as "new". So, it could well be the new tubing you had had maybe been used for something else? Then returned and unscrupulously sent back out to you as a new product by a shady third party seller. Unlikely, I know. But, maybe it's possible?
The sandstone could be the culprit too, being pourous maybe it picked something up and then seeped it back into the water? Again, unlikely if it had been stored safely and used in a tank previously without issue.
Maybe in the move you lost your cycle on your filter media? Another unlikely one though, because it'd have to be dry for a while for all the bacteria to die.
Could also be oxygen depletion. I know in low o2 situations plecos will gulp air from the surface to get by. Your oxygen could be low for a few reasons. If you do a large water change and your tap water contains chloramines, then the dechlorinator requisitions oxygen from the water as part of the chemical reaction that does away with said chloramines (an actual chemist could explain it better than me haha). Also, maybe you had a heater malfunction, which would also cause a big die off due to low oxygen. Although, you'd likely notice the water extremely warm to the touch for that to have been the problem.
When it comes to toxins in the tank, the question a lot of the time is not if a particular substance is toxic, but rather in what concentration is it toxic. I'm not sure of something commonly found in houses that could accidently end up in the tank, in enough volume, that could kill everything but the pleco like that. I've had people spray nasty smelling air fresheners right next to the aquarium and worried for days, but noticed no ill effects.
Without more info, the kind of info that's difficult to ascertain honestly, you might never know what caused the deaths unfortunately. It goes without saying but if you suspect some sort of contamination, I'd break the tank down, scrub it with a blue dawn solution, hose it out and start over.
Maybe somebody else on the forum has some better insight than my few speculations there, but it's all I could think of.