I have a gourami which has developed a lesion on it's forehead. The lesion is white and raised with a 'lip' around it.
It was first noticed ~10 days ago a day or two after a 30-50% water change. One of the gourami was noted to frantically swim against the corner of the tank during the water change. I'm assuming it was this one. The lesion has slowly grown in size since then.
There have been several additions of fish and plants to the tank lately (in the last two months) without quarantine. The gourami were the last to be added 4-6 weeks ago.
There are two other gourami and about 20 tetra, none with lesions.
The affected fish is behaving, swimming, and eating normally.
I guess an infectious disease is possible, but no other fish are affected. I looked at an online guide of infectious diseases and none of them seem to be a match.
I'm hoping that this lesion may be due to trauma. I've hoped it would get better, but it has actually gotten worse. Maybe the initial trauma left him susceptible to an opportunistic infection that the other fish aren't suffering from because they didn't traumatize their skin.
The tank is a 55g with undergravel filter. I have liquid test kits for pH and ammonia, which are zero and 6.8-7.0. I have dipstick tests for other parameters, which are normal. The temperature is set to 76F.
I'm most concerned if I need to immediately remove him due to an infectious threat to the other fish, or if it's ok to 'wait and see'.
I'd appreciate thoughts from anyone else.