Ever try the 2 liter soda bottle trap? IME It works extremely well at catching every single fish in the tank EXCEPT what you are after, ha ha. Maybe that's just my luck and yours might be different.
Take a 2 liter soda bottle, cut off the top 3 inches just past where the curve starts, then invert that piece back into the bottle. Put some food inside and gently submerge it in the tank. I use a binder clip to hold the inverted top in place so it doesn't float away when submerging it. Fish swim in through that little hole, but find it very difficult to get back out. Don't leave it in the tank for more than a one or two hours at a time tho since there isn't any water circulation inside the bottle and uneaten bait inside will start to decay. Sometimes it works great, and its worth a try.