its certainley not as easy as i thought it would be lol
when i first tried, i took about 20 and ended up with about 3 that were minimum standard, that link will tell you all you need to know, i have read it,
The only way to take fish pics without a flash is to have really bright lights on the tank. In fact if you can get more light on the tank the flash won't upset the fish as much.
Unfortunately if you slow the shutter speed down you will get blurry pictures.
The other option is to increase the ISO setting on the camera. The lower the ISO number the finer the detail on the picture. Raising the ISO number from 200 to 800 or even 1600 will allow you to shoot in darker conditions without a flash. The drawback being the images will not be as finely detailed. Try it and see, you might be happy with the results.
When you aren't using the flash reduce the F stop number. Take it down to about F4.5 or less if you need to. The lower F stop number will allow more light into the camera.
the biggest thing to remember is to take LOTS of pictures if you are using a digital camera. You will get one or two gems for every 20 or 30 pics you take.
put all lights on tank<turn off room lights,,,,set camera on a table or something similar line up what you thinks a good shot,set the clock tick tick tick SNAP......remember to use macro i find this helps