Hoping someone can give me an idea what could be going wrong…
65g tank setup since January, seeded with a sponge filter from an established tank. All inhabitants doing well, mostly pea puffers, a few danios, otos, one hillstream loach. Up until this weekend had an egg crate light diffuser for the top. Saturday made a top out of lexan polycarbonate. Glued hinges on using regular super glue. Water parameters stay stable, no ammonia or nitrite, nitrate around 20ppm. Sunday morning wake up to dead danio. Tested water, all good. Later in the day do weekly water change and discover dead hillstream loach. After water change find dead pea puffer. Monday morning another dead pea. The remaining puffers seem fine. The remaining two danios are breathing heavy so moved to a different tank on Sunday. After Sunday water change added second air stone and propped open new lid thinking maybe it was an oxygen issue? Other equipment in tank: HOB, sponge filter, heater.
No other signs of disease on fish. All have been wormed, peas are fat and happy. Could the new lid have anything to do with the loss or is this just a coincidence?