I had a common Pleco in my 55 Gal tank w/ 3 fancy goldfish (ORANDAS)
They lived together almost a year in good harmony- but once the Orandas got hand or fist size, the Pleco would grab onto the fish and suck-
and the spot it would suck would get a small dot of blood to the surfice-, we have a Fish Dr Here in the Atlanta area, I called to find out if this behavior was normal? and was told about the plecos having a taste for the slime coat of gold fish.
When I stated this above, it was not without fact., No a pleco is not a vampire, but they will suck on the side of larger fish- in my case, my Oranda Goldfish.
Needless to say, I took the pleco out, and just for ther record I now only house 2 fancies (Orandas) in my 55G- I gave one t a pal.
I completly agree with the 20 t0 25 Gal rule for per Goldfish!