Too small and/or too few fish in a too-large tank commonly leads to this kind of feeding problem when the fish are first introduced. I get the impression that if we ask how long these fish have been in this tank the answer will be measured in single digits of days?
If yes...patience! They just need some time to learn the ropes in their new environment, including where food is to be found. A few swords or other livebearers...who will settle in very quickly...will demonstrate this to them and can then be removed if you wish, although in a 55gallon the swords could likely live with them indefinitely. Depending upon the age of these convicts, they could be many months away from forming a "breeding pair", whereas the monkey-see/monkey-do learning phase will just take a few days.
Is the tank decorated/planted? If not, providing some cover for the convicts will make them less skittish and more likely to explore and feed when food is present. Floating or slow-sinking, i.e. more neutrally buoyant, food will remain obvious for a longer time, especially if there is a current to move it around a bit. Avoid the temptation to add too much food, which will simply spoil and pollute the tank.
And, sorry, but I had to chuckle when I read the part about getting a pleco to "do the cleanup for me"; that's a bit like worrying about your kids dropping food crumbs on the floor of your home, and deciding that the solution is to get a non-housebroken Great Dane to clean them up.
