What is this?

[BenA]

This evening while feeding the fish I have noticed that one of my Melanotaenia Nigrans suffers as of something that looks as the gills on one side are out and the fish breath faster than the rest. What do you think? Can I do something about it?

Sorry for the poor photos.

Thanks.

[xXInkedPhoenixX]

Did you test for Ammonia? It's hard to tell are the gills inflamed and only one side or both? Anyone else have redness? Could it be an injury?

[BenA]

No, didn't test for ammonia. It is red and seems to be out of the gills cover on one side only, also, the only fish with this anomaly. Last weekend I have changed about 60% of the water and about 25% today.

[xXInkedPhoenixX]

Test your water parameters and post them if you can. It might just be an injury, keep a close eye on it for any changes. You could put it in QT and treat with a bacterial med or aquarium salt to prevent bacterial infection if it's an injury, but since we still don't know what it is it's hard to say for sure. I would test the water first and go from there. 

Edited November 13, 2021 by xXInkedPhoenixX
to clarify

[BenA]

It is too late for ammonia as I have changed  about 30% of the water yesterday. In any case i attach better photos.

 

[Colu]

That looks like a growth I would Qarantine and treat with aquarium salt 1 table spoon for 2 gallons and an antibiotic treatment such as kanaplex or maracyn2 see how he goes

[BenA]

Thank you Colu.

This was also my conclusion. The fish died overnight and I have looked at it with a X10 glass. The gills were one lump and didn't have any blood in them. I talked with the LFS and was told that rainbows do suffer of tumors. 

[Colu]

Tumors are very common in fish 

[BenA]

This is something that i didn't know. I wonder if fish in the wiled suffer the same.