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Definitely manual focus. It is better to have your camera/lens pre-focused on a certain area and have the fish swim through that area. Lighting plays a huge role as far as how much it allows you to increase the depth of field, and where it allows you to dial in your shutter speed. If you remember from the presentation, I said you want to be roughly at 1/40-1/60 of a second at least. This helps freeze the discus if light allows.