I'm sorry for your loss. How big were your Snakeheads and did they vomit their food before death? I had very beautiful juvenile Giant Snakeheads last year with awesome personalities. I fed them prawns from the market and they vomited them later and died. I never feed prawns anymore, they must have so much of bacteria and parasites.
I'm not sure about the tuna, were they fresh from the market? Water levels or sudden temperature changes can stress them out also but sudden deaths raise more questions.
Although I believe that Giant Snakeheads are designed to eat a lot, in captivity however I feed them every other day. Sometimes I fast them for proper digestion. Besides, it also prevents ruining the quality of the tank water and creating any possible diseases. Imagine throwing raw tuna into the aquarium on a daily basis. And if your Snakeheads were little babies they are less resilient.
And Giant Snakeheads are messy eaters, unless you have some suitable bottom feeders that will not be harmed. This is not a problem in the wild.
My current Giant Snakeheads at 1 year old :