You don't have a camera pointed at your tank 24/7?? What kind of responsible reef keeper are you?
Just kidding!!
Let's ignore 'whodunnit' for a moment. Maybe it was an angry neighbor... Some will poison your dog if they poop on their lawn, etc...
First, can you describe 'crash'?
What was in it before? How was it doing? And what died? Fish and invert or just inverts? What Survived? Smells? Visuals? Water color? Strange sounds?
You want help investigating a potential crime - but you give us no clues (murdering someone's pet is a crime- yes)
Personally, if the perp knew about chemicals, they'd dump hydrogen peroxide in the tank at lethal levels. It would wipe everything out, then turn to water and oxygen. Untraceable.. evil...
Then again, maybe you are really the 'wife' seeking revenge and I just gave you some very bad ideas... Paranoid much? LOL