If your doing it for the love of plecos, you should have them in an appropriate sized tnak, and doing a better water change routine. Give them an environment they can thrive in, not just survive in.
I agree, get a larger tank and some plecos that stay an appropriate size, then you won't have to return them after two years. I have a blue phantom in my 55, it stays 6 inches max and is awesome! I'm hoping to get a butterfly soon. There are plenty of awesome plecos that stay small(er), there's really no reason to get a common you have to return in a year so it becomes someone else's problem.