What is this??

[Atk3and2]

What are these spots on this rainbow? None of the other two rainbows have it, nor do any of the other fish. This fish has had these spots for months. I’ve treated the tank with erythromycin, ich-x, general cure, with no effect.
65 gallon tank, nitrate 0, nitrite 5ish, ammonia 0. Been running for 2 years.
I’d like to add new fish to the tank, but I want to fix this first.

 

[Fishfur]

Looks like a case of Lymphocystis to me. It’s viral, therefore not treatable. So all that medication was unfortunately pointless.

It’s an Iridovirus which is contagious but most fish that get it recover eventually without lingering after effects.

It’s pretty rare for it to be fatal unless the tumours show up internally or interfere excessively with the fish being able to eat or possibly swim.

It’s wise to practice the best possible husbandry when fish have this virus because any viral infection has an impact on the immune system.

Having an active viral infection makes the fish more susceptible to any other pathogen it might encounter.

 

[Fishladyforlife]

I have a rainbow with lymphosistis. I had 6, now I am down to 2 due to this disease. Here are some pics of them right before they died.

 

[GlennO]

The fish in both cases appear to have mycobacteria infections. Unfortunately, rainbowfish are rather prone to it, which is why I no longer keep them. These external ulcers are the most common symptom. It's very difficult to cure and will eventually affect internal organs. My suggestion is to euthanize when symptoms appear and only buy rainbowfish from trusted breeders and not big box stores.