Fish dying despite apparently fine water parameters

Question

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

1. Nitrate - 20ppm
2. Nitrite - 0ppm
3. pH - 7.0
4. KH - 120ppm
5. GH - 180ppm

Answer 1

It seems you've put a good amount of effort in making the tank parameters ideal, but not every home aquarium and LFS tank have the same parameters. How did you place the fish in the tank? The most common way is to place the fish bag in the tank to help livestock accumulate to the temperature, but there are other factors as well. Another way to place the fish in the tank is through the drip line method. This method slowly mixes the tank water the water the fish was in the LFS, making sure it becomes used to other parameters like PH. You should try this with the next fish you buy.

Another factor could be water changes. You mentioned your local water is a bit hard, and your tank water has a ph of 7. What methods are you using to decrease your PH? Since it seems your corydoras are a bit sensitive, I recommend making small water changes at a time. If you want to be extra careful, change the PH of the top-off water before you add it to the tank.

You mentioned a problem with one molly eating all the food. With the flakes, just try something different. If the molly is also eating the pellets for the corries, maybe you should try distracting the molly with flakes first. Otherwise, buy some veggie tabs for the corries since they can't be eaten in one bite and usually shared.

I wouldn't be too worried about the death of the first molly. They're weak fish and bound to have one die out of a batch. I also don't recommend using test strips. Just get the API master freshwater kit and you're set for half a decade.