There isnt a lot of aquascape to that tank. Nothing to break up lines if sight so a fish can keep away from an aggressive fish.
Not that it relates to your current issue, but goldfish and plecos shouldnt be kept together. Plecos diet tends to change as they age, and they often get a liking for fish slimecoat. Fancy goldfish make a tempting target as they arent very agile and are slow moving.
Do you know what kind of pleco it is? Goldfish are temperate water fish, some plecos need warmer water. They generally need driftwood in the tank so they can rasp on the wood which aids their digestion. Plecos grow to a variety of sizes depending on their species. From a few inches upto potentially a couple of feet long. Common plecos are commonly sold as babies as a solution to algae growth, but they get upto 2 feet long and need a tank 4 x bigger than the one you have.