I use stock Canon digital Rebel with flash, mostly using the Portrait mode, either auto, or manuel focus.
The camera needs to be pointed slightly downward to avoid reflection.
Tanks lights on, or sun. Although sun is tricky.
A black background helps these type shots, I tape a styrofoam panel spray painted black to the back of the tank
Glass should be very clean, outside and inside the tank (algae easily ruins shots).

Below a couple sun shots

For very small fish, I add a micro ring lens

I usually take about 100 shots at a sitting and if I end up keeping 2, I consider that successful.
I have never been able to get a decent shot with a phone.
