Overdose of Flourish Excel

Did something stupid, overdose flourish excel into my fish tank. Why did I do such act? There left a bit of liquid in the bottle, so just pour every single drop in it, i believe the volume is more than 15 ml.

The observation:
Fishes started to swim on the water surface, grasping for air.

The effect:
Fishes suffocated and died.

All the neon tetras in my tank died except the 2 hard and strong Oto survive the ordeal. So sad as these tetras have grown alot, some up to 1.5 inch long.
How the Oto survive? They have suckle that helps them to hold on to the tank's wall. I saw one of the Oto, stick their mouth above the water surface and breathe heavily.

However, this do not mean that flourish excel is no good, it is good for all the living creatures in the tank when you use it at the right quantity.

My planted tank update 2

Little O2 pearl on the rock moss.

O2 Bubbling Java Fern.

Cherry shrimp near the surface on my drift wood.

My cute little Otocinclus.

More about Otocinclus:

The perfect little catfish for the community aquarium the oto will do a fine job at keeping the algae in check. Small to large aquariums will benefit from their service as this fish bothers none and is usually left alone.