I suppose the question is how to make them grow faster, no? I put mine in the outdoor pond, I've never fed them, they only eat algae and always look fat, and after a year, they grow from 15 cm to around 50 cm, I hope they;re still growing.
If your water parameters are stable you pleco will grow fast. Feed him cucumber, zucchini, lettuce, peas, corn, they love and need their vegetables. Algae waters and shrimp pellets are also good.
i was gunna say, 20 gallon is way to small for a common pleco that can attain 18-24 inches.. get him a tank of atleast 18 to 24 inches WIDE ( try a 120 thats 4 foot by 2 foot by 2 foot ) a 20 gallon is way to small. if you cant get a bigger tank then sell him and get a bushynose pleco or rubberlip pleco