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Currently my tank is 20G Long with a new Fluval 205 with 2 RES about 1 and half inches. I was thinking about adding a Pleco to spice up the tank a little. I bought 10 feeder fish and 2 goldfish back then and none of them has become dinner yet for my 2 turtles. Currently I do not feed the fish so they probably pick up scraps of turtle food and I dont plan on feeding the Pleco. What do you think?

In general, a bad idea. You've already got a huge bioload in your tank with 2 turtles, 10 feeders and goldfish (which are usually as messy as turtles). Plecos poop more than they clean.
The long fins of the pleco may also be too much for your RES to resist, and they might just nip the pleco to death.

i had 2 large plecos in attempt to clean up my tank...

epic failure. hank nipped at the tail and fins until they couldnt swim away, and well... you know what happened next.

The fish will become food at some point.I went through 10 plecos,5 cory albino catfish, and countless chinese algae eaters till i learned my lesson.
The only fish i keep in the tanks now are catfish that i catch at the pond.I have one that's 10" right now and keeps the rocks VERY clean.

I have a pleco (Mr. Sucky Face) living with my two RES (Frankie and Fergie) and have been for almost a year. My turtles don't attack my pleco. In fact, it is usually the other way around. Sucky loves to give Frankie a good squeegie and she doesn't seem bothered by it at all.

I do however agree that pleco's won't do anything to help clean your tank. But I like having him in the same tank. IMHO it is a nice and different addition to the tank.

But again, my turtles are wierd, so if you don't get the same results, it is not my fault haha.

Very bad idea in a small tankl.

Your tank is too small / got too much in it already. Adding a pleco will just make situation worse.