Tuna has mercury. https://www.menshealth.com/nutrition/a19520234/is-eating-tuna-every-day-safe/
impaired brain function is just one of the possibilities from mercury poisoning.
Well, be careful. Anything under 1k could cause your body to go into starvation mode. Also, you certainly do not need to lose weight, so there's not much reason to be consuming less calories.