I believe that's what this is, it has been growing rapidly. I'm not sure what to do with him. I have had him 7 years so I know that's pretty old for a yoyo loach. But its starting to look like its about to rip through the last layer of skin.
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Water tests are all good. This is a large red mass.I have one with an iceberg tumor but looks nothing like this. Poor fish. It may be bacterial. What are your water parameters and how often do you change water? Is there a lump? For now, I would do a 75% water change daily. Order some API Furan-2 and Seachem Kanaplex. Use both as directed. Until then come continue to do the daily water changes. You can possibly save him if you move quickly.
Could you give us the numbers? Sometimes all good is not what it seemsWater tests are all good. This is a large red mass.