I just checked the EXIF on your aquarium shot. The lens was all the way open and the shutter was at 1/8 second. Light that looks OK to your eyes isn't necessarily enough for a photo. 1/8 is as low as it goes in auto and it obviously needed to go even slower. Unless your Canon is a DSLR you would have had the same problem with your Canon assuming maximum aperture is f2.8. You needed fill flash for that shot. If the Canon goes slower in auto you would likely have gotten a blurred shot.
The histogram should have been shifted to the left. With better light you could have used the EV correction to increase the exposure. The left and right buttons are defaulted to EV. For that shot it would have alerted you that you needed fill flash.
Shifting to manual and increasing the shutter speed only works if there is an increase in aperture available. Otherwise the shot will be too dark. Once the lens opens all the way you can't increase the shutter speed and still get a good exposure.