[you2]

You asked for fast lenses ... and on MFT, you effectively lose 2 f-stops due to the sensor. I, for one, love the G9 (& Panasonic colors).

I suppose few people can give you an answer as to how the AF will perform in your specific use case though.

yea i guess fast is relative after all i've been using a phone camera which i find quote slow and focus in-accurate. You do need a little shutter speed to freeze thing else there is some bluring even if the focus managed to be accurate; i'd think f2.8-f4 with 1/250 to 1/500 is probably good enough; faster lenses are sometime useful for isolation if the fish is in a crowded field; but there will be times you want a phone like aperture for good depth of field - so maybe f8. I just don't know how good the panasonic g9 auto-focus is - i do like the idea of the sony even if it is over-kill but not really the idea of a large lens collection. A zoom is really useful for this sort of thing given the fishes constant change in depth (most of my aquarium are now 4 feet front to back and it is not always that the fish will be near the front); and once you move to full frame you start staring at monster zooms.