Major tetra die off

[Mtl Parrothead]

I’m a little confused with what happened. (My daughters tank)

Here are the tank specs:

Cycled since March
26 gallon
Temp 78.7
Ammonia 0
Nitrite 0
Nitrate 10
Ph 6.8-7

Stock(as of yesterday):
3 APISTOGRAMMA SP. VIEJETA GOLD + 1 fry
5 neon/cardinal tetras
2 Amano shrimp
MTS snails

Plants:
Java moss
Java fern
Anubia

I noticed a week ago something white looks like a scratch on the head of one of the apistos. I began a Pimafix and Melafix treatment. Today was the end and we were preparing to do the water change. When the lights came on today it was pretty cloudy but I didn’t think much if it. Everybody was swimming and seemed good. After lunch we went to do the pwc, all the tetras were dead and I couldn’t find the fry or the amano shrimp. The apistos seem to be fine. We did about a 10 gallon water change, none of the tetras seemed to look sick or had any visible signs of infection or trauma.

Any clues what could of happened since the parameters seem to be good.

Thanks

 

[Mtl Parrothead]

Edit:

I’ve found the amano shrimp and it’s normal that I sometimes don’t see the fry for days.