Neon Tetras Dying - No Identifiable Symptoms... Any Ideas?

[gabriel.mihu]

One fish dead 7 days ago, one fish dead today. They look good - size, roundness, colors, fins, everything; oldest ones about 1 year, newest ones about 6 months old.
 
Tank size: 36g moderate/heavy planted, eheim 2215, co2 20ppm
pH: 7.6-7.8
ammonia: 0
nitrite: 0
nitrate: 0
kH: ?
gH: ?
tank temp: 74F

Fish Symptoms (include full description including lesion, color, location, fish behavior):
No symptoms as far as I can see, the only thing that seems a bit out of the ordinary is that they're hiding in the plants a little bit more than usual. (like if they were searching for cover 10 minutes/hour, now thy do that 15-20 minutes/hour... just putting numbers out  there)

Volume and Frequency of water changes:
15% every two weeks
Chemical Additives or Media in your tank:
Seachem flourish and some potassium/nitrogen fertilizer - been doing this for months.
Tank inhabitants:
9 tetras, 10 guppies, 4 black skirt tetras, 3 corries, 1 platy
Had a few galaxy rasboras temporary, for about a week, but most of them disappeared, maybe consumed by the big guys.
Recent additions to your tank (living or decoration): none

Exposure to chemicals: mentioned before (fertilizers)

 

[AshleyNZ]

I would just add that you should up the cory's numbers to 6 or more

 

[gabriel.mihu]

Well, currently trying to find a new home for my 1 green cory and 2 beautiful sterbai cories. The green cory seems to be lonely since I lost his pair while moving the tank.

 

[gabriel.mihu]

All my neon tetras (11) are hiding in the plants shade, coming out only for feeding a little bit. They went from schooling nicely and being happy in the tank to hiding in the bushes.

 

[gabriel.mihu]

50% water change, one more dead, no change - all the neons stay hidden between the plants.

 

[gabriel.mihu]

I just saw one of them fighting to stay afloat - he keeps going down like a stone and has to swim to go up or even to stay afloat.