A couple of tips for those with a DSLR.
Most modern SLRs have the ability to change the focus point when shooting. The old habit of using center focus and re-composing will usually end up in a blurred or distorted shot when shooting through tank glass due to the refraction. Try to shoot head on to your subject and use the focus point adjustment to compose your shot.
If your camera has spot metering and focus adjust it to use spot focus but use the averaging setting for metering. Unless you understand the zone system using spot metering will usually result in part of your image being too light or too dark.