High aperture is great for landscapes but lenses lose sharpness when you go past a certain point (like f8). With aquariums, you need the speed to capture the fish/shrimp so I would turn it to shutter mode and go for 1/125 or 1/200+ faster. (This of course depends on how your camera's iso performs and lens speed but regardless, if you need to stop fish from moving you need faster shutter speed so no matter what lens/camera i use, I find myself going to shutter speed for aquarium photos.)
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