archie1709 - if you can, bump up your camera's ISO setting. Default on a digital is probably 100 - you could try 400, 800, 1600. Higher settings will be grainier, but will let you get away without flash.
Then use a shutter priority, and set it at at least 1/100, I prefer 1/120. That's a fast enough shutter speed to catch them if they're moving.
If using auto-exposure, your aperature will open up as wide as it can, but it still may be a dark photo if you dont' have really bright tank lighting, so a little adjusting in photoshop later might be needed.