Eek,, just seen your signature and seen that he's in a 20L tank? You really need to get him a new tank asap. I cant tell by the pics what species he is, but if he's a common plec he needs to be in a tank 4ft x 18" x 18", which is 75g. Commons can and should reach 12" in their first year, reaching 12-18" in total very soon after. Your goldfish also needs a much much bigger (and IMO seperate) tank.
In a 20l litre tank, a goldfish and a plec are going to make so much mess that they are literally swimming round in their own crap. How often are you doing water changes, and what kind of filter do you have running? What are the ammonia/nitrite/nitrate levels? Is the tank cycled?
You need to get them out of there asap - if you want to keep them together, then the 4ft x 18" x 18" tank will do if he is a common plec. I would seriously consider splitting them up into two (much larger) tanks or rehoming one though as goldfish are coldwater and plecs are tropical, so if you keep your plec in a coldwater set up he wont be as healthy/happy as in tropical, and if you put your goldfish in a tropical (heated) set up - his lifespan will be halved, and the mess will be unbelievable.
If he's a bristlenose, it might be worth posting on the coldwater board to find out the minimum tank size for a goldfish, I'm not 100% sure what it is - but I'm thinking it'll be around 30g.
Can we have a full pic of the plec please so we can work out what species he is?