What could I put in freshwater 55 gallon tank?

Plecostomus are not good companions for goldfish. Many plecostomus can grow huge and many like to suck on the soft slime coat that goldfish have. Plecostomus are best suited to central american cichlid tanks and south american cichlid tanks. I recommend you return it to the store and purchase some aquatic snails instead (such as apple snails) to control algae.

Your tank should be able to hold 3-4 fancy goldfish. Fancy goldfish = fantails, moors, orandas, ryukins, lionheads, ranchus, pearlscales, pompons, veiltail and demekins. This gives you a wide range of goldfish to choose from. I recommend you keep another goldfish with your current one as goldfish are social animals and like to be with others of their own kind.

Comets, common goldfish, shubunkins and other types of goldfish are not fancy goldfish and so are incompatible with the fancy goldfish which they are likely to outcompete and bully.

I recommend not having a plecostomus and goldfish together but it does work out for lots of people. Your tank is big enough that as long as you manage the water quality they should be good the only thing you really can add would be another goldfish.

LOL---See your other question for this answer too.

I answered it there . Hadnt seen this Q yet