Goldfish - Black On Fin

Hi All
Know this forum is for tropical fish so hope you don't mind me posting this query about goldfish.   

I'm looking after my neighbour's goldfish while they are on holiday & I noticed the fancy one has a black line on its dorsal fin.  Could you give me any advice please? 

PS excuse the photography - I have read the posts about taking photos!
PPS yes, the tank is too small for these fish but this is actually a bigger tank than they were in before due to me moaning at the neighbour to get a bigger tank for them!

It's the first time I've noticed the black bit but I don't see them very often so maybe I'm mistaken.  I don't think it has a heater, just the filter.  My neighbours are back next week so I'll mention it then & just keep an eye on it in the meantime.