My Dalmatian Molly died, can anyone help me figure out why?

[Lucrezia90]

I had fishes die before, either eggs or disease, but this is inexplicable!
So, today we went to the pet store, got 2 aquatic plant to add to my 5.5 (found out they gave us 3, but anyway), so I did the weekly water change around 40%, I cleaned thoroughly and then added the 3 new plants (2 El Nino fern and 1 potted betta plant anubias sp, the one in the picture). Filled up the tank and fed the fishes. They were fine for about an hour, one (the dalmatian, Ziva) chasing the other (a sunburst platy, Tony) playfully, nothing different than usual. So I went for a nap, and 2 hours later Ziva was with her "mouth" on the gravel, all contorted and wasn't moving anymore.
I can't think of ANYTHING, other than perhaps he hate some of that yellow filling in the plant pot, but it seems unlikely... Tony is looking fine, so if it was a disease he would have been dead or showing symptoms too anyway, right? It's not water, cause I JUST changed it, like a couple of hours before!
My head hurts trying to find a reason why. (I took a picture of Ziva, out of curiosity, thinking that it might have been the eggs, (my last betta died of eggs) but it wasn't.)
Can anyone help my out, please? Thank you.


If it helps I have the usual, heater, filter, water bubbles that go off in the morning, 2 fake (ceramic?) logs and I do the weekly 30/40% water change.

 

[SouthAmericanCichlids]

Maybe the plants had some disease on it, but I don't know of any that hit that quick. And the cotton thing in the pots aren't harmful.

 

[bettafairy]

The only thing that sticks out to me is the cotton in the plant pot, as I’ve been told to remove it and add different substrate whenever I’ve purchased those pre potted anubias. I’m not sure if the cotton really is that harmful, but if it looks like a large portion of it was eaten it could have created some kind of impaction?? What are your water parameters?

 

[Lucrezia90]

The only thing that sticks out to me is the cotton in the plant pot, as I’ve been told to remove it and add different substrate whenever I’ve purchased those pre potted anubias. I’m not sure if the cotton really is that harmful, but if it looks like a large portion of it was eaten it could have created some kind of impaction?? What are your water parameters?

I was talking with someone when I bought the new plants and even though he said he had some himself and said these were ok, he never once mentioned to remove the cotton! Just out of anger and sadness, how was I supposed to know?
I checked the water.
PH 7.2
Ammonia 0,
Nitrite 0
Nitrate 20
Not sure why the spike in the nitrate, since I do the weekly water change pretty regularly. Could have been the new plants? Also, could that have been the reason why Ziva died?
Cause if not I'm out of options...

 

[WRWAquarium]

Sorry for the loss of your fish

I have never heard of the material in plant pots being dangerous before but I guess its possible if a small chunk was consumed.

Im going to say that 20 nitrate is not what killed your fish. Even if it spiked somehow suddenly I doubt that would affect a molly.