Hi and welcome to R2R.
A couple things of note is the size of the tank, honestly that is too many fish for that size of tank. There is no where for more passive fish (fire fish) to hide or get away from more aggressive fish such as clowns or aggressive crabs and shrimp.
Another consideration is diseases, fish housed in too small of a tank and in a newer tank may become stressed and that allows some of the more common diseases to set in.
The open brain played no part in the fishes demise, it was just coincidental.
Generally the majority of clowns will show some aggression even if it's just posturing. Fire Fish/Dart Fish are shy and initially reclusive, they would not put much of fight against a dominate clown.
For future reference, members are going to want to know tank chemistry, Ammonia, Nitrites and Nitrates, so it would behoove you to test for those.
The learning curve is a large one in this hobby. We want everyone to succeed and enjoy the hobby. Please feel free to seek any kind of assistance you need on R2R.