Fish suddenly dying , dont know why

Tank
What is the water volume of the tank? 60 liter
How long has the tank been running? 4 months
Does it have a filter? Yes
Does it have a heater? Yes
What is the water temperature? 26 celsius
What is the entire stocking of this tank?
5 platy
8 neon tetra
1 halfmoon betta
1 cpo
1 ramshorn snail
Maintenance
How often do you change the water? Every two weeks, depends on nitrate levels
How much of the water do you change? 30-50%
What do you use to treat your water? Stress coat+
Do you vacuum the substrate or just the water? Yes

*Parameters - Very Important
Did you cycle your tank before adding fish? Yes
What do you use to test the water? Jbl stick + ammonia alert
What are your parameters? We need to know the exact numbers, not just “fine” or “safe”.
Ammonia: 0
Nitrite: 0
Nitrate: 25
pH: 6.8

Feeding
How often do you feed your fish? Once a day with a break once a week
How much do you feed your fish?
What brand of food do you feed your fish? Jbl
Do you feed frozen? No
Do you feed freeze-dried foods? Yes

Illness & Symptoms
How long have you had this fish? 4 months
How long ago did you first notice these symptoms? A week ago
In a few words, can you explain the symptoms? 1 platy was lethargic and didn't eat for 3 days, died after that, no visible symptoms, might have been a little bloated.
1 neon tetra died toady, do nd the corpse, didn't notice anything suspicious before
1 neon tetra was swimming funny (couldn't balance himself but was able to swim to where he wanted), eutonasied him
Have you started any treatment for the illness? Microbelift herbattana
Was your fish physically ill or injured upon purchase? No
How has its behavior and appearance changed, if at all? The neon tetra that was swimming funny became bloated 3 monthes ago

Explain your emergency situation in detail.
Above, ill add that i didnt find any visible parasites and that the tank had a little nitrate spike a week ago (reached around 80 ppm)

 

This is weird. 2 questions, has anything from other tanks been added to yours recently and what's a cpo?

 

1. Didnt add anything new, last addition was a coontail a month and a half ago.
2.cpo= mexican dwarf crayfish

 

sometimes crayfish prey on small fish. Did you notice any signs of the dead fish being attacked?

 

No, its a mexican dwarf, he is way too weak to attack anyone and he is super afraid of them
The neon was eaten, but i dont think its the crayfish fault, the platy was intact
The cpo been there for more than three months, and nothing happened
The cpo been there for more than three months, and nothing happened
P.s the tank is 6 months old, not 4, my bad