Catching fish is incredibly easy with only an inch or two of water in the tank, believe it or not ;)
Pump the water out into reservoirs, catch the fish, re-fill. You won't have to move any rocks, disturb any animals in the tank, and any corals (if you have them) will be fine during a short 'low-tide' period.
The only complaint I've ever heard with this method is not having large enough reservoirs to hold your water, which in your case being 400 gallons would obviously require one huge reservoir or several smaller one's. This is easily and affordably overcome if you don't have the reservoirs on hand: pick up a few Brute cans from Lowe's or HD and return them when you're done.