I know that nitrate is naturally in the tank with cycling, but I changed the water anyway (sorry, i read yorupost afterwards). I was worried about it going higher because of the pleco remains. I also have some biomax in there for at least four months. The tank was given to me and the biomax was used before for I don't know how long so that might have contributed to problems. I'm going to check the water again later, on Sunday Or Monday, and see how it the water is.

As for the medication, I put in the Internal Paraside Guard by Jungle, one of my fish has stringy transparent poo, one of the symtoms.
After putting the medicine, didn't change 25% like it told me since that would be too much water change within three days, and the fish seemed okay.
One of my goldfish seems a bit skinny, maybe it's the beginning of internal parasites, not sure.