Help to ID an unhealthy growth

Hello.

We've had 3 cories for a while in a very healthy, planted, and well-cycled tank.

One started to develop a growth on its gill, and it does not look fungal (i.e. no fuzzy texture). This was about 2 weeks ago. Since then, we have quarantined it and have treated the water with the following at the directed intervals and doses:

Jungle Fungus Clear
API Melafix
Fritz Coppersafe

This fish is usually in a school of 3 cories, and the other 2 appear healthy with normal behavior. The quarantined fish is also behaving totally normally in a 1 gallon quarantine tank at 76°. It has been eating and digesting without issue, too.

Does anyone know if this growth is potentially a tumor and, if so, should we just return the fish to it's usual habitat?

Thanks in advance for your time and help.

- Greg & Jess

Gill tumour. There's no cure so put it back with the others. When the fish has trouble eating, swimming or breathing, euthanise it.

Colin, thanks for the advice.

It is much appreciated.

- Greg & Jess