When I had my 400, I used to have to drain the tank down. Also, as the other poster said, fish are pretty groggy/slow when its been dark for a while. I've been able to scoop up fish with that sneak attack method. LOL.
I read recently about an ingenious method but can't remember where. Basically it was a fish trap made from a water bottle. Cut the top part of the neck and invert it into the lower part. Make sure the hole is only large enough for the small fish to get into. Put some food in the bottom of the bottle so the small fish are interested in getting to it. There was more discussion but you'll have to look for it.
There are youtube videos that show the method cgjedi mentioned. Not sure how effective they will be to get out 100 juvies though. I'd lower the water and use big nets. A second set of hands with two more nets will make it way easier. I recently helped a friend to catch juvies in a 180g. We used 4 nets and had them out in no time. Can you temporarily rehome the Arowana?