[toni-a]

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.

An advice from an aquarist and fish preeder with 5 plus large tanks and photography enthusiast: for fish behind glass forget about full frame and the high resolution thing, shoot any black and white thing inside the water and check yourself  for the massive CA you have and the slight tilt of the focus plane that must be 100% parallel to the tank glass will destroy all details.
I don't want you to waste  your money and get nothing in return.
In my own experience I had better results from smaller sensors, aquariums require patient, forget about sports photography style autofocus.
One last thing, make sure to clean algae from the interior aquarium  wall, even the tiniest amount you don't see might cause issues.
If your aquarium is curved, forget about shooting close ups of fish from outside altogether, you can however shoot the aquascape however of course.