This man was trying to find suitable spawners for the workshop practical. Because these catfish can use atmospheric oxygen via their accessory breathing organ, he let all the water out of the tank to make it easier to sort, but unless he shuffles his feet carefully while in this tank, he may accidentally step on the spines on the doral and pectoral fins, not something you would want to do. BTW, removing all the water may not kill these tough fish right away, but it will stress them, so working through this as quickly as possible to achieve what you want do is in order.