Hey! There are few things you should remember:
1. Try to get "in front" of your subject. This will make your AF system work faster and allow to get rid of fancy chromatic/geometric abberations caused by bowed glass.
2. Adjust the "sensitivity" to be at least ISO200 with max ISO800! Experiment a bit with that! Higher ISO = better sensitivity + color saturation!
3. Avoid taking photos with external/back light on! Reflecting light on the surface of your bowed glass will make AF go crazy and cause auto-exposure metering of your camera choose the wrong program, yielding under- or over- exposed photos! Use native (tank) light only!
4. If you got image stabilizer turn it on, however pictures taken from the tripod should be shoot with IS-off!
5. IMPORTANT if you shoot a picture of fish or coral use SPOT-metering method! It will give a priority to the center of your focusing system, making the AF-adjustments faster and more accurate!
I can imagine your frustration. I spen few days scratching my head and figuring out how to make best use of my camera and lenses.
Here are some of my shots:
