[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.

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.

Ok thanks. I guess the G9 might be an option then - i'll read some more reviews. I have 4/3 lenses include the 28-70f2.8 zoom but my body is the original om-1.

[toni-a]

Do post some photos here, you know aquarists how do they love seeing fish thriving.
One of my best looking fish (a red spotted severum) is a total nightmare: among others I own 2X severum a specie known to be on the peaceful side of aggressive cichlids, first one is docile and peaceful, the other one is a total headache it constantly bullies its tankmates, it uprooted all the plants and chew on the leaves of the ones he can't uproot, it destroyed all decorations in the tank, my tank was half sandy soil, the other half , planted aquasoil , with some rock separation between the two the severum literally ploughed the tank and carried sand upon aquasoil and black aquasoil over sand.
If it weren't beautiful with vivid colord and good to photograph, it would have been gone long ago.
BTW you would ask why I didn't put him in his own tank. in fact I did but last month my blue acara spawned and had to move them to his tank and moe him back to the community tank, negotiating with my wife if I can get a fifth tank, she said, you can't have both either the new lens or a new fish tank ......

[you2]

Do post some photos here, you know aquarists how do they love seeing fish thriving.
One of my best looking fish (a red spotted severum) is a total nightmare: among others I own 2X severum a specie known to be on the peaceful side of aggressive cichlids, first one  is docile and peaceful, the other one is a total headache it constantly bullies its tankmates, it uprooted all the plants and chew on the leaves of the ones he can't uproot, it destroyed all decorations in the tank, my tank was half sandy soil, the other half , planted  aquasoil , with some rock separation between the two the severum literally ploughed the tank and carried sand upon aquasoil and black aquasoil over sand.
If it weren't beautiful with vivid colord and good to photograph, it would have been gone long ago.
BTW you would ask why I didn't put him in his own tank. in fact I did but last month my blue acara spawned and had to move them to his tank and moe him back to the community tank, negotiating with my wife if I can get a fifth tank, she said, you can't have both either the new lens or a new fish tank ......

The problem with posting pictures here is they have to be such low resolution. I have mostly peaceful sa cichild; like festum and soon geo winemilleri. I want to get some krobia. In the 550 (10 foot long) I have a school of clown loaches (13); the first week i had them they swam the full 10 foot day and night for 2 weeks but now they have settle down and mostly just come out when i feed the angels or the lights go off.