PlanetCatfish.com

So, I've bought this cat fish, which I believe to be a Pterygoplichthys parnaibae, but I'm not sure. I think it has 12 fin rays, so I'm prety sure it's a Pterygoplichthys. If you could just tell me for sure what kind of pleco it is, i'd be really grateful

Considering that is commonly bred in captivity, where the P. parnaibae isn't commonly available at all, it seems more likely that it is the former. In medicine there is a saying of

If you hear hoof beats, look for horses, not zebras.

If there are two options, where one is common and the other very unusual, then it's most likely the common option - it may be wrong, but it is much less likely than the one that is unusual.

Rio Parnaiba is from south-eastern brazil, which is not really where most fish in the hobby comes from. If this fish came through an unusal channel, then it most likely would be found in a specialist shop, and you would already know what species it is, because it would be correctly labeled in the tank, and have a price to match.

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Mats