Not to be argumentive, but based on the size of fish in ratio to the tank in the video...that can't be 300 gallons. It "looks" like a 75 gallon maybe, which would equate almost to an 300 liter tank? By the way, your tank is beautiful!
As for what is causing the die off, my thoughts are add a bubbler to break the surface of the water, and also what temperature and pH is your tank (sorry if you already discussed this)...I'm just thinking this is possibly due to low available o2. Oxygen, temperature, and pH are linked.