Tiger Barb marking change

I have 3 tiger barbs which I’ve had for a number of years, however, in the last month or so 2 of the have started developing dark black blotches. Could these be caused due to changing an external canister filter at about the same time, and is it treatable?

Those fish are not tiger barbs, they are black widow/black skirt tetras (different names, same fish). But I'm afraid I don't know if the markings are something that is normal with the tetras or if it's something to be concerned about. But changing to an external filter shouldn't cause it. Having said that, if you didn't use the media from the previous filter and the tank went through a fish-in cycle when you changed the filter, it is possible the markings could be a response to ammonia and/or nitrite in the water.

This is localised nerve damage causing a malfunction in the melanophores (the cells that produce.. or don’t... dark pigmentation). It can be caused by injury, bacterial infection, or a tumour. If the black areas are expanding over time it could be melanoma.
You could try treating with a bactericide but usually there’s nothing you can do.
It can be a temporary condition, with the fish reverting to normal after a while, and it isn’t always life threatening for the fish.
It’s nothing to do with the filter. These are not the symptoms of ammonia- or nitrite-poisoning.

Thanks for the insights guys. I did use the media from the previous filter albeit the previous filter was out of action for a couple of days before I was able to get a new one, so I’d assume that any bacteria on the old media would have died by that stage?

As long as the media was wet, the bacteria would still be alive.

But as I've never kept these fish, I would listen to Ichthys that the markings are not caused by ammonia or nitrite.

This condition isn’t common, but it’s often seen as a blackening of one side of the head, which I believe is often caused by a bacterial infection. It doesn’t seem to affect the fish. It’s a different outcome of course if it’s a tumour or melanoma. It doesn’t seem to be contagious usually, so the fact that you have an outbreak of it suggests to me that it could be bacterial. But who knows? I haven’t seen any studies on this, so all we can say is that the nerve endings that control the melanophores have been damaged by something.

Transferring the mature media was the right thing to do. They won’t die in two days without food as long as they were wet and had oxygen, and when switched on again they would just resume as normal.
Even if a few died, which can happen if the media are literally full of bacteria due to a large bioload (eg big fish), they can double their numbers in a day so they’d soon pick up.

I’d treat with a course of bactericide on a ‘why not?’ basis, but there’s nothing else you can do but the usual looking after of the water...

They’re actually a really nice shape for ‘a number of years’ old ‘tiger barbs’. Black Widows nearly always end up obese, but these are still sleek. Well done. (maybe I just feed too much....)