Identify white stuff

This seems to be a recurring thing, it goes away then comes back a few days later. I don't know what could be causing it as there is nothing in his tank

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Maybe some more photos, is there any erosion of the fins, or is it superficial?
Did you treat it etc th anything last time?

 

Nope after 2-3 days it goes away but it has now come back for the fourth time since the first time

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Looks like a fungus.
How frequent and big are your water changes?

 

Every week 50% I'll try get a pic tomorrow as he is at the office

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