This is my first post(apart from introducing myself).
I have recently bought a 75 gal. tank and transfered my existing fish into it (4 of them).I have since added 6 new fish (gradually).One of the last fish I added,a beautiful oranda has a white lump on his dorsal fin.It has not changed,none of the other fish have any signs of anything and the afflicted fish(named brains by my son) is well in every respect apart from this.
Can fish have deformities like this ? Does a lump or spot always signify disease?
The fish has been in the tank for just over a fortnight.
Thanks for any help you can give.
Wendy

is there only the one? if there is only a few just on the dorsel fin i think it could be breeding stars. have a look on the gill plates for any more white dots.

Fish don't get breeding tubercles on their dorsal fins, just their pectoral fins and their gill plates.
Is the spot on one of the rays on the dorsal fin? If so then it could have gotten damaged somehow and the white spot is just where the break is. I have a fish at the moment with a spot like this on a bent bit of his fin where he must have knocked it.

If the white spot looks like the pic enclosed then it would be a broken lead ray
on the dorsal fin. Not much you can do except make sure your water params
are excellent and maybe dose with some Melafix to aid the healing.

Good luck!

Thanks all for your answers.Pm94 ,that picture is exactlly what it looked like.However since writing the post this morning the spot has disappeared.....is that possible?!
Thanks again.
wendy

From what I understand the white spot is basically a scab. The spot
where the ray broke will never grow anymore though. I've got a
couple dudes like this, but they don't care. They still tear around
like a fish normally does.