hi,im new to keeping oscar and other tropical fish.i bought a baby oscar when he was only 2 inchs.i have a 20 g tank which i keep him in.other tankmates include a redtail shark and pleco.i had a tiny albino oscar in too but he died,i duno how?and my tiger oscar ate him!! i also had a siamese fighter which was also eaten by my oscar.is my redtail shark and pleco in danger??

The pleco should be fine. As far as the shark, I'm not sure, but I would imagine that if it is non-aggressive towards the oscar, and is quicker than the oscar, it should be ok.

OTOH, oscars have been known to annoy other fish to death, by constantly chasing and nipping at them. If your shark appears to have bites taken out of fins, or you notice the oscar constantly chasing it, you may want to either remove the shark, or get a few more, to dilute the attention among several.

Problems will be made worse because of the size of the tank. You really need to think about a new tank as soon as you can.