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if you are not on manual and the camera is selecting this aperture then you don't have enough light to go to a smaller aperture (smaller aperture is a bigger number one like 8-11) so if you bring down the iso your camera will select a slower speed as it cannot go with a wider aperture than 5.6. your lens I guess can open till 3.2 or 3.6 when at the widest setting and this will decrease as you zoom to reach 5.6 at 300mm.
Try to see what values you get when all your lights are on and if the value is still 5.6 then yo ushould increase the ISO to more than 800 which will start to give some noise in the pics. you could either try a slower shutter speed if you're suing a tripod. for corals you could be fine using as slow as1/30 or even slower if they're not moving with the current. maybe shut your pumps to decrease their movement. one other alternative is ot use flash an external one would work better than the built in as you can angle it up and to the side to avoid reflections. flash would work better for fish shots where you need to shoot at least at 1/60 1/90 to freeze their motion. for orlas flash unfortunately washes out the colours of some corals so it's not always a good idea to use flash.
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