Fish Behaviour

[Fish 4 Life]

Hi im Chris i am new to tropical fish.
I was given a red tailed shark and a pleco, they got on really well until i introduced another pleco into the tank with some tetras. The red tailed shark did not go for the Tetras but he went for the Pleco instead. I am confused because the Pleco is a lot bigger than the Red tailed shark and i got told the Red tailed shark will not attack anything bigger than it self, but my new Pleco has 2 pink flesh marks where the Red tailed shark attacked him.
Can you tell me why the Red tailed shark attacked the Pleco or could he of heart himself? (There is no spiky or sticky out bits in the tank where the Pleco could have court him self on)

 

[iSpeakWhale]

its possible that your shark could of attacked him due to territoral issues. i had the sam sort of issue in my tank. i had catfish and gouramis together for months and they were fine, however when i introduced two more cat fish the gouramis began to attack them and the other fish too. Strange!