We have a 40 gallon (150 liters) tank with an established nitrogen cycle. We went several months with just 1 three-striped cory and 3 gold dust mollies.
The tank has its own filter that came with it rated for its size, a heater, gravel substrate, 3 large rocks, a piece of driftwood and half a dozen or so live plants.
Recently I purchased 6 more corydoras as they are meant to be kept in shoals. In the past three weeks 4 of them have died without any indication of stress or disease. They were all dark, not too thin, foraging and swimming regularly. Water parameters have been tested regularly and nitrogen cycle has been in expected levels, pH and KH are ideal, and the water is just a touch hard (due to the area).
We recently added 4 more gold dust mollies and 2 tangerine mollies, and after about 5 days one of the tangerines died, again with no signs other than little fits of wiggling (possible symptom?). One of the female gold dust mollies from the original bunch is roughly 3x the size of any of the others and very aggressive in hunting down both flakes and pellets at feeding time, and we're worried other fish may not be getting enough to eat. Is this possible? How might we get food in so the corys can forage without her eating it all? Is there something else we might consider?
Edit Measured Using API 5 - 1 Test Strips
- Nitrate - 20ppm
- Nitrite - 0ppm
- pH - 7.0
- KH - 120ppm
- GH - 180ppm