Went back and reread your thread, you have some stocking issues that have been covered already, but many experienced goalkeepers on here go by the 10x rule (meaning for your 55gallon tank 550 gph water turnover).
Goldfish imo should only be kept in very large tanks or preferably an outdoor pond, do you know anyone with goldfish pond?
Plecos can be big polluters, what kind are they.
These two factors would probably be the cause of your problems, and even with proper stocking 10x filtration is no guarantee for water quality, only water changes do that.