It's quite a good film, Kate Beckinsale is very watchable in it............

Beat me to it!!!
Hi Jon,
I think it's going to be a case of trial and error until you find the method that works for you.
It's something that I've done quite a bit of, but my method is very different to Russ's.
I tend to avoid using a high ISO, as with the dark backgrounds noise becomes an issue with the 450D. Aquarium lighting combined with the need for a high enough SS to freeze the movement
is a problem though, so I tend to use the 50mm wide open and rely on my technique and reactions to get the shot.
Of course, nailing the focus at F1.8 isn't easy especially with small fast moving objects. (not to mention being hampered by shooting through glass and water), but it is completely doable with practice. These days I usually get around a 75% keeper rate.
My other tip is to have the tank lights on, but draw the curtains and darken the room as much as possible to minimise reflections. Get up as close to the glass as possible, and position yourself at a slight angle to it rather than straight on to avoid getting a reflection of the camera in the shot.
A couple of my favourite shots below. These were from my 6' tank so I was shooting through 8mm glass. Some more
here if you want to have a look. (EXIF data intact on Flickr)
IMG_0086 by
SarahLee1001, on Flickr
Shoal of Tiger Barbs by
SarahLee1001, on Flickr
