Do you happen to have a large spare fishtank / large container (e.g. a 145-litre Really Useful Box) plus filter and/or airstone that you could move it into? It might be that it is less vulnerable in there and can be treated more easily. Does it look injured or bloated in any way?
If injured (perhaps by a bird), then it might be that it requires anti-finrot treatment. If bloated, it might require fasting or being fed peas (with shell removed) to help reduce the bloating until it's well again.