Hi,we have 3 lovely large Fantails,in a large tank,and recently these white spots have appeared on one of them,there appears to be various causes of this,possibly Ich?...Anything I can do please?

My two thoughts are either whitespot/ich or breeding tubercles. The latter usually occurs on the fish's gills, head and pectoral fins, though - not the dorsal fin and tail as here. Is the fish chasing one/both of the other two and engaging in rough breeding-like behaviour? If not particularly, then I'd say this is more likely whitespot, in which case these articles might be helpful:
https://www.fishkeeping.co.uk/articles_88/white-spot.htm
https://www.practicalfishkeeping.co.uk ... a-problem-like-whitespot/

Thank you 🙂👍,very helpful.