It is possible the pleco is showing signs of stress. Both those fish will need to go into a much larger tank very soon. That oscar needs at least a 60 gallon aquarium. They can reach about 18". That is a whopper fish that needs a whopper tank with whopper filtration. They can grow to half that size within a year or two. Growth then slows down and may take 10+ years to get full potential size.
Common plecos in the wild can reach 3 feet in length. They live longer than oscars and get much bigger and therefore grow slower, but it will become a very large animal.
A 20 gallon tank is just WAY TOO small even if it's used just as a rearing tank prior to a larger tank. The oscar produces more ammonia than the average fish and both are waste machines. Feeding these guys properly is important to keep good water quality, but the volume of water plays a key role in the upkeep of the water's quality. The less volume of water, the quicker it fouls. The water quality looks good now, but for how long?
Stick to not feeding live feeders to the oscar. It's not healthy. Goldfish feeders are the equivelant of candy and can harbor nasty bacteria and parasites. It only takes one to make the oscar sick, so just because it hasn't happened yet, doesn't mean it won't. Thawed out meaty foods like krill, beef heart, and silversides are good substitutes to live food.
Give the fish a time limit in feeding. For an oscar, three minutes worth of food per day is enough to maintain good health with an occasional snack of thawed out frozen food. This will help keep the nitrates at somewhat controllable levels.
Plecos should be fed at night being they are nocturnal feeders. It's good the oscar is getting it's veggies though
It's important for all fish.
The plants will be trashed from both the oscar and the common pleco regardless how big the tank is. Oscars dig, uprooting plants and the common pleco will eat the plants. Smaller species of plecos like the bushynoses and thomasi plecos are plant safe, but the common pleco and many other larger species are not plant safe.
I say you either need to trade out the fish for ones that will fit in the tank or start cycling a 60 gallon tank or larger for these guys to live their lives in. Keeping them in the 20 will result in deformities if not territorial outbursts of deadly aggression kill them first as they grow. Fish that are still growing continue to grow, but to the shape of the tank and this can cause some ghastly deformities.
There is some evidence, though not sure if formally documented that poor water quality and poor nutrition can dwarf fish growth, but the system itself is not healthy and the fish generally die prematurely.
