Mackenziem

Hi, my goldfish isn't doing to well and is just sitting at the bottom of the tank in the same spot all today and yesterday. I am in the cycling process and bought prime to switch to from my old water conditioner as I know it detoxifies ammonia. I thought that there was a chance he was stressed from the ammonia (even though it was only 0.25ppm), so I did a 50% water change yesterday and dosed the whole tank with prime, and left for 24 hours, and dosed the tank again as there was still 0.25ppm present. This, unfortunately, doesn't seem like the problem so it's a bit stressful as I'm new to fish keeping and have no idea what is wrong or how to fix it!! Here's some info I've gathered so far: -he's still eating if I drop a pellet in (don't want to feed him too much yet seeing as he's sick) -he is flashing occasionally, and rubbed himself on the substrate a couple of times -extremely lethargic, not doing anything except grazing the bottom for a minute or two -he's new, and think he may have caught something at the pet shop?? -normal poo, same colour as his pellets and not at all white or bubbly just incase you want to know about my water parameters, ammonia 0.25ppm, nitrite 0ppm, nitrates 5-10ppm. Had him since the 19th of November.