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It's hard to say from the picture. Its most likely a bacterial infection of some sort. True fungal infections don't happen very often. First thing you need to look at is the condition of the tank. Water parameters, what fish and how many of them. Stressed fish are more susceptible to disease and can be harder to cure. Bacterial infections can be cured with something as simple as lots of clean water and some salt (depending on the type of fish) but more serious infections may require antibiotics. If you can provide us with some more details of your tank then it will help with diagnosis.
Thanks we done a water teat and everything ok apart from ammonia is a little high so everything ok on that aspect and I can't see no stresses with the fish all seem happy And ive seen no chasing so I think fungal