Nice looking corals! Looks like everyone already provided some great advice. But a few other thoughts might be to try shooting as perpendicular to the glass as possible to minimize distortion. As already mentioned I would also suggest that all water flow is turned off, you shoot between 30- 50mm, stop down to around f8 and keep the iso on the low end. Shooting on your tripod will let you use as long of a shutter speed as you want. If you don't have a shutter release you can use a timer instead. Focus manually because sometimes autofocus misses because of the water's distortion. You should also look into getting some kind of photo editing software. Photoshop is glorious but expensive, alternatively you can look into Adobe Lightroom or Gimp shop which is free. All of these programs can process raw photos and will allow you to get A LOT more out of them.