parasites?

[Rondelet]

Popeye is caused by a lot of things but it's most commonly due to a bacterial infection, physical trauma, ammonia toxicity, or gas saturation in the water. I doubt poor ventilation would lead to gas saturation. Check your water pumps to see if any of them are sucking air in - as this could led to saturation of gas. Even so, it would be fairly rare in a home aquarium. You need to get your water quality sorted out - best done through lots of water changes. You may also consider giving your fish an antibiotic treatment via the feed if possible. I would advise against added antibiotics directly to your water for a variety of reasons, but mostly because 1) efficacy is poor; and 2) they will upset your biological filtration; something that would exacerbate your situation if your nitrogen cycle gets thrown off and ammonia starts to build up. I would try administering an antibiotic via the feed, which is a more efficacious route of administration. Any of the Seachem (Focus + any of Sulfathiazole/Kanaplex/Neoplex) or Aquarium Products (eg Gel-Tek Ulta Cure BX) would be good choices to consider.