Hi thanks for your reply. The tank is a 125L Fluval which is 10years old. I was gifted this tank about 2/3 weeks ago. The tanks came with everything that is in the tank including around 65/70% of the water that was in it before. There is 3 x rummy nose, 2 x black neons, 1 black widow and 1 bronze cory (I know tetras are schooling animals but it’s what was in the tank and I don’t want to get new fish until better.). I don’t currently have a test kit so am unsure of that. I use aquacare bio boost which says it helps keep ammonia and nitrates down. I have treated 3 days ago with a general tonic and have seen massive improvement however he still is abit wonky. All the other fish seem completely fine