Algae wafers: First of all, I don't quite agree with Bas on the "low protein" - I guess that's a relative term, but the Hikari and Ocaen <whatever> ones that I've got are around 35% protein, which isn't much less than your average flake or pellet food. It may be lower than "carnivore pellets", but the "generic" foods are around 35-45% protein. I do agree that they contain fibres.
I wrote an article on various "home-made" foods for Planet Catfish, if you want to feed a pleco, you may want to read it. Here
As Bas infers, it's the protein in the food that breaks down into ammonia and thus leads to nitrite and nitrate.
I still think feeding sparingly, both plecos and cichlids, is the best approach until your nitrite is in the less than 0.5 ppm [mg/l] range.