Turn the lights on (if there's an easy way to do it) to full white. That will allow the camera to stop attempting to 'fix' the picture. With the brighter lights, the camera will use a faster shutter speed which means less motion blur. Also, when the light is low, in an attempt to keep the shutter speed as fast as possible, the camera will bump up the ISO, which will directly result in added graininess we see in the first two pictures.
Barring that, the other thing I've done is, with the lights off in the tank, point a bright flashlight at what I want to take a picture of.
If your phone has the ability, you could put it into full manual mode and adjust the shutter and ISO on the fly, but that's likely more work than it's worth.
One other thing, because the camera is using a slower shutter speed, you need to keep it as still as possible. Hold it up against the glass, brace it on something, hold it close to your body (as opposed to holding it out in front of you). Anything you can do to reduce camera shake will help. Another tried and true method is to use the timer (meant for group pictures). Not having to push the shutter button means you don't shake the camera as it's taking the picture. The short countdown before it snaps the shutter on it's own gives you a few seconds to steady yourself.
