Mass die off

[Opie3001]

I lost 9 fish in one night and I am not sure what I did wrong.

 

I had a fish with one Redeye Tetra pop eye. So I added some API Melafix and API Stress Coat+ and API Aquarium Salt. All meds per the box but I was light on the salt part. One tablespoon per 7 gallons. The box says one per 5 gallons

Did that early in the day. 

I watched them all day and everyone seemed ok.

I was more worried about all the snails and plants but not the fish.

This is stuff I have done before.

 

Well this morning I found 9 dead fishy's. My two Danios, two Siamese algae eaters, two pink tetras and rest were smaller tetras.

I have no clue what I did wrong. These fish have been in this tank for 3 to 4 years. The tank is heavily planted looks like a jungle.

Tested the water yesterday and again this morning when I found them nothing wrong.

Plants look great even the snails all seem on and look good. 

So I took out the bodys and did a huge close to a 100% water change and now I am waiting.

 

Oh by the way the red eye tetra with the pop eye is still alive and seems ok so far.

There are about 20 other fish in the 75 gal tank.

 

Water Parameters:

• pH = 7.2
• Nitrates = 10 or less
• Hardness = 150 ppm
• Nitrite = 0
• Ammonia = 0
• KH/Buffer = 120 ppm
• Water Temperature = 77

Edited December 12, 2021 by Opie3001

[Colu]

It's possible the medication caused low levels of desolved oxygen did you add an extra air stone during treatment

[Opie3001]

I have a canaster filter and a air sponge filter and a baby groot air stone in there. Both air ones are turned up so there is alot of air.

I am also about to put in carbon in the canaster to filter anything out.

I would think with two air stones it should be ok???

[Colu]

It sounds like you have enough airation other than a reaction to the medication but  it might just be a coincidence that you had the mass die off after adding the medication nothing else is jumping out at me