There is not much you can do with the cell phone in terms of settings at least with my note 3 there isn't. I normally keep iso at 100 or 200 and I drop the exposure level near the lowest setting. Darker is better for my as I can lighten up the photo after in post.
The largest factor is available light. I always wait till late at night to take photos. Kai is less hyper than too so he is still a bit longer compared to during the day.
The rest of the room.needs to be very dark to avoid reflections. Blacked out is best if possible. The tank should be lit up nicely though and play around with light position to find what works for you.
Another huge thing to consider is stability of your.phone. make a tripod or use something as a rest for when you taking photos. Could be anything like a stack of books or whatever. Just a flat surface to rest the phone on to help keep things from.shaking too.much.
Sometimes I even push my phone right to the glass. When away from.the glass I like to take pics on a slight angle to avoid seeing many reflections.
Another thing to pay attention to is that each park of the tank will product a different photo because of the lighting in that area and what else is around to shade it. So experiment with different areas and use food to help get him into the spot you want.
Finally you need to have some sort of post process software aka editing app. I like aviary app. I use it in this order:
1. Sharpen the image - don't overdue it or it looks really bad
2. Adjust brightness
3. Adjust contrast
4. Adjust exposure
5. At this time you can make small tweaks to things like vibrance, shadows, saturation, and things like that. But generally I like to not mess with the colors as I want to keep it looking natural and mimic what I see irl.
I forgot to mention that you would or may want to crop the photo before beginning to edit.
That's all I can think of for right now. Hope it helps