[APD] Sudden death of all the healthy yamato shrimp

["Ng Heng Ghee" <THGNg at ntu_edu.sg>]

I have a 2ft (80Litres) planted tank that is 6 month old. An external Eheim 2213 was used for filtration. 
The lighting is 2 x 36W PL tube, 6 hr of illumination each day.

My stocking density was:
- 80 Glowlight Tetra
- 30 Yamato shrimps

A few day ago I decided to redo the layout. No water change was done. 
I used back the existing plant and some from another tank.

My new stocking density is:
- 50 Yamato shrimps
- 10 Malayan shrimps
- 15 Cardinal tetra
- 10 Harlequin tetra
- 5 Rummy nose tetra

After some feeding, I switched off the light at around midnight. Both shrimps and fishes look fine and active. 
No distress was observed.

But the next morning at 7am, I found some fishes and all the Yamato dead.
The fish are breathing frantically at the waterline.

The water test shows:
pH: 7.3
kH: Not tested (there are 2 handful of coral chip in the filter)
GH: Not tested
NO2: 0.3 ppm (due to the decay of the shrimp)
NH3/NH4: 0 ppm
Cu: 0 ppm
Cl: 0 ppm

After that I did a 50% water change and added AMMO-LOCK for water treatment.
I left the light on for 8 hr before going to work. When I reached home, the fishes were still showing sign of distress.
They were breathing at the waterline.

So I connect an airline with an airstone to provide more oxygen to the fish.
It works. 

Could it be an Ammonia or Nitrate spike ? I used fertilizer like KH2PO4, K2SO4 and KNO3 without ill-effect.
Could I have caused a re-cycling of the beneficial bacterial ?
Why is the fish breathing so frantically ? What cause the suddenly depletion of oxygen in the water ?
What killed the shrimps ?

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