Here are a few tips for yall.
1. Tripod of your not already using one.
2. Compose your pictures using the rule of thirds. Be sure to not have distracting backgrounds or areas around the tank unless its used for framing.
Someone said the pictures should come out in true to life color but thats not true at all. The sensor in your camera can sense colors that we can not and then bring them out just like film. Especially if you are shooting around night time or with the lights off and the aquarium light on you can get some sweet colors that you will not normally see.