Am out of my bailiwick when it comes to investing, but thought this article on the safety of the market (that ETF's are supposed to enhance), might be worth a read.
"ETF Losses Today Were Far Beyond What The Most Sophisticated Risk Models Could Have Predicted" | Zero Hedge
It deals with the situation that can occur when the amount of cash redemptions at a trading desk exceeds the allocated at risk capital. It can essentially halt trading, and cause the very steep market drops.
Most of these problems occur because of the high frequency trading where automatic trades take place in milliseconds and ahead of the human reaction time.
This morning there were announcemnts from exchanges that purport to fend off the "crash" effect, but the pundits question these changes.
A harum scarum article but something to keep in mind as it describes what actually happened yesterday.