I wouldn't decrease your shutter speed just yet, you got decent exposure there with ISO 80 and F8.0 at 1/4second. Going slower will only let you decrease (higher f number) your aperture from F8 which will give you more DOF but I would see what you get with proper focus first. I've shot over 1 second exposures before without the need for mirror lock up so I wouldn't bother with it until you believe it's actually causing an issue but if you want to try go nuts, won't do any harm.
You can shoot manual exposure but it might be easier to shoot in aperture priority (Av), start at F8 and adjust your exposure +/- to get a shutter speed that gives good exposure. This is similar to manual but sometimes a little easier.
When I say manual focus this is different from manual exposure. I see now that you're using a Canon PowerShot A590 IS so not sure manual focus is an option. But if it is, it's manual way of setting the focus point which is normally done automatically when you push the shutter button half way down.
Also set you white balance WB to custom and set it to your tank color by setting it while focused on something white/grey (sand usually works).
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