I use a Canon PowerShot G3. I think they're up to the G7 by now, but it's the same basic camera. The auto settings are excellent, and it gives you the option to go completely manual as well. The macro setting also allows you to get extremely close.
As far as tips, there are a few things that help me:
1. Use a tripod. It allows you to concentrate on the subject and not on holding the camera, plus it reduces blur from camera shake.
2. If it's intense enough, the tank's lighting almost always looks better than flash.
3. Shoot a lot, try lots of parameters, and delete most of your pictures.
PS. I have moved this to products and DIY, because it seems a little more suitable.