With only one flash it is best to shoot as you have been doing from the front at enough of an angle to avoid bounce back.

Yes, that's right, Larry!
Thanks again for the comments, Folks!

So, let show off more Teeth again...

Would mind sharing the camera settings you use with the flashes that render such exquisite details?

My Camera is CANON 350 D
Lenses: EF 50 mm/f2.5 Compact-Macro and EF 50 mm. f/1.8 II
Ext. flashes: Canon Speedlite 430 and 580EX

My usual settings:
Shutter Speed: 1/200 seconds
Aperture Value: f/14-22(dependent on the fish specific and fishes position)
ISO Speed Ratings: 100-400(dependent on the fish specific)

My Speedlite 430EX is always situated above the tank(slave mod)… the other one - Canon Speedlite 580EX (wires connected with the camera in “master” mod) I hold in my left hand and move in front(like straight “fill” flash), dependent on the fish specific and fishes position…
The both are with diffuser!

Thank you for all the information about the settings and equipment you are using.
I have an Olympus SP-320 I bought new in 2006 for only $300.
Now it sells for $454 on amazon.com. First time I have ever seen any electronic product appreciate in price but it has some features lacking on most digital point and shoot cameras. It has threads which allow the use of auxiliary filters and macro lens. Like most of the point and shoot cameras, it does not have interchangeable lens capabilities.
I am thinking about getting a new camera like the Olympus E-P3
with the kit lens and the MSC M. Zuiko Digital ED 14-42mm II R f3.5/5.6 Zoom Lens. It is one of the new micro 4/3 cameras which have features matching many DSLR but are still relatively compact cameras.The prices overlap those of many DSLR's.
I did buy a couple cheap Zuiko slave flashes which I haven't tried out yet.

At between F-14 and F-22, it is no wonder you are getting such great depth of field in your macro photos.

Just want to remind you all, Folks, about our present KAB Killi Exhibition’2011 in Sofia; BULGAIA(1-2 October).
http://www.petshop-zoomania.com/Mix/Poster%20KAB%202011.jpg
Here is one of the Handsome Fellows you’ll see there - Terranatos dolichopterus

Some more of my Lampeyes Killifish:

Procatopus similis Kumba GPE 90

Here You may enjoy in bigger photo resolution with my lovely Fundulopanchax sjoestetdi Niger Delta, covered the June' 2012 issue of TFH Magazine:
http://www.tfhmagazine.com/images/COVER_T0612_1920.jpg