Planted Tank Guru
Join Date: Dec 2017
Location: Elk Grove, California
I was attempting to find an environmental stressor that would be working to lower the fish's immunity-- allowing bacteria and fungus to get a hold of fish fins.
Not sure I found it.
I would up water changes minimally from what you are doing now- putting yourself at no more than 20ppm nitrates by end of week. Not sure how often you are doing water changes now; but, I would recommend at least 25% weekly for long-term health of fish. Neons are very sensitive to rising organics in water.
For now, because they appear healthy otherwise, I would treat with API's Fungus Cure ( active ingredients are malachite green and acriflavine) or Paraguard ( which is basically the same active ingredients but with the addition of methelyene blue). Make sure that between each dose that you change at least 25% of water. These aquarium medications work at their most efficient the lower the organic load is in water column. The more water you change, the more effective the uptake of medications. This also has the dual benefit of clean water allowing the fish the ability to build own immune system to fight illness.
What are you feeding and how often?