I wouldnt be surprised if the plecos ate all the snail eggs when they were laid, so the snail population eventually died. One problem with my plecos is that they eat live plants too. I have an albino pleco that eats plants, but I love him because he doesnt hide during the day and eats algae very well.Strange..my old tank had a snail problem, but my once I redid the tank their population is stable.
My old tank had gravel, and some people I know with a overpopulation with snails have gravel, a planted substrate or a rocky substrate.
Once I redid the tank I filled it with sand.
Its very well possible substrate has something to do with this.
I keep dead plant matter around for the shrimp and plecos. No snail explosion.
However, I have no algae.