Things to look at could be if you're shooting at the tank from an angle the glass will distort the image, another consideration would be raising the ISO so you could raise the aperture as well. With such a low aperture yes you gain light but you also loose focus depth meaning only a small portion of your picture will be in focus i typically use an aperture of 9-11 which restricts light entering through lens, this is fixed by using a high ISO but not too high because "noise" which basically refers to too much pixelation due to high iso in a dim lit area will occur. your shutter speed is pretty fair at that speed but you could for sure drop it even lower to allow more light into the sensor. Just keep your hands steady to avoid shake or be creative and find something to pro yourself against, photographing corals under blue light is in itself a challenge lightroom photo editing software helps a lot you can adjust the white balance to help remove the blues in favor of a more appealing color balance, you can also use noise reduction dials to allow you to shoot with a very high ISO of course this is all dependent on your light someone shooting under LEDs has much less light to work with when compared to T5 so work with it but to sum up this mess i just wrote slower shutter/9-11 aperture for more of the coral in focus/higher iso without including Noise. If you'd like to shoot under pure Blue lighting id advise getting an orange or red dive filter you can find them online by simply typing in the filter and lens size on google if you choose to use these filters having lightroom would for sure help in changing the white balancing in post production
