The key is to lower the water level until they can't move much, they lose the will to fight and splash once they no longer have their free space to swim, then they will have no choice but to go into your net. I also find that they will do so willingly, perhaps they feel its the only wise choice to make once the water is taken from them. Water is their power, once you take that away, you have all the power, it works. You'll want an extra net on top to keep the fish from jumping. I've done it so often, I got a good knack for it now. My oscars use to put up a fight, now they just lay into the net, think they know it's going to be alright. I was surprised to learn, one of the reasons a fish will put up a fight is because we ourselves are stressed when trying to catch them, remain calm and your fish will too.