Fish dying! Please help

I have a problem and need some help!

Here are the details.

Tank Specs:
30 gallon planted Freshwater Tank
Water quality good
fully cycled for over a year
Fish Stock as of Saturday:
1 Betta
6 cherry barbs
6 neon tetras
2 amano shrimp
5 Cory Cats
1 Coolie loach
All have been in the tank over 6-7 months.

I wanted to add a few more plants, tetras and corys.

On Sunday I added the following
1 Anubis
1 Water Sprite
1 Borneo Fern

on Monday I added:
6 neon tetras
2 Cory Cats

I thought everything was fine until today. I went down to visit and my Betta,2loach, 2 tetras, and 1 Cory cat are ALL dead!

I'm freaking out. Anyone have any idea what is happening!? Please help.

Test your water immediately. There might have been an ammonia spike.

And then do a big water change

Agree. At the first sign of any trouble with fish, do a major water change. Provided the parameters--these are GH, pH and temperature here--are the same, this cannot do any harm.

When you say "water quality good," presumably you have some tests. Always post numbers so we know what they are for what you can test. The pH, ammonia, nitrite and nitrate are advisable tests for any problem. GH and KH will presumably remain fairly stable.

I would suggest something may have arrived on the fish or plants. Just a thought, without the test/data.

Need pictures of the dead fish if you have any. Also pictures of the remaining fish.