Sorry. That's probably not the answer you were hoping for, but with 4 goldfish plus tropical fish in such a small tank the problems are only going to escalate. If you want to keep the goldfish and raise them to maturity you'll need a 45+ gallon tank for fancies, and if they are comets or commons they'll eventually need to live in a pond.
Reticulated Hillstream Sucker (Sewellia lineolata).
Keep in mind that with these fish you can't have many tropical fish, due to temperature differences. Fish such as white cloud minnows would work however.
Aquarium Pleco Fish, Plecostomus Catfish Species Care Guide
Hypostomus plecostomus (known colloquially as a sucker fish) is the scientific name for a type of freshwater tropical Central and South American fish belonging to the family Loricariidae. They are large algae eaters, and to differentiate them from small algae eaters, they are often referred to as plecostomus, often abbreviated as plecos or plecs.
Although the plecostomus is a known common algae sucker, it requires more than just pellets and tank algae. It should also be fed algae wafers, zucchini, cucumber, lettuces, peas, melon and also any semi-soft fruit or vegetable will do as well.
Hypostomus plecostomus - Wikipedia
Sucker fish like a well-decorated tank. Although they may interact with other fish in the tank, a pleco is most comfortable with plenty of places to hide. Intricate ceramic aquarium decorations that tend to collect algae are practical if you have a sucker fish in the tank; he'll keep them polished. A tank that receives natural sunlight through a window for part of the day and consequently grows algae will supply the sucker fish's need for fresh greenery and also give him a chance to bask for short periods of time. Add at least one piece of driftwood to the tank; sucker fish like to chew on wood with the rasp-like teeth that surround their mouths. Keep water temperature in the same range as for other tropical fish (68 to 82 degrees F) and be sure the water is moving and aerated by using filters.Generally calm and nonaggressive, a suckerfish is typically a good citizen of the aquarium, as long as he’s the only one of his kind. He’ll get along well with most other tropical freshwater fish. However, if he outgrows the tank and feels overcrowded, he may become aggressive. Let a suckerfish be the only burrowing fish in the tank; if he has to compete with others for his burrowing spot, he can become aggressive and fight fish such as knife fish, rope fish or eels.