If your shooting with a digital slr you can use a macro lens with a hood and put the hood right up to the glass. That will prevent flashback from the glass. You could also use a diffuser or an omni bouce to keep from getting that direct blast of light. 2 main things can make your pic look grainy , the wrong iso and noise. The higher the f-stop you use the longer your shutter needs needs to be open to let enough light in. To the original poster nice job.
macro lenses are the way to go, but if we are talking true macro, not these zoom things they call macro. you will need quite expensive extra gear, to give even illumination at such close distances, you will need a ring flash, or something similar. these have to be mounted right on the end of the lens! close focus lenses, the zoom jobs they call macro, are a different thing, you need to be nowwnere near as close to your subject, with these. if you can get yur flash flush with the side of yur tank, it makes no difference what angle your camera is, you will get no flash back.
and for the life of me i cant see what shutter speeds have to do with pictures taken with flash
?? true the sutter speeds for flash is normaly set to a standard speed, this is to avoid the shutter being seen in the picture, but if you set a lower speed, it will have no effect on your picture. flashes fire at any thing up to 500,000 of a second, whatever speed you set, within reason, it will not be noticed, as the flash has frozen the motion anyway.
you can use what is called slow sync, this is a short low power burst from the flash, this freezes static objests in the picture, then the shutter will stay open allowing the ambient light to add it effects. but unless you want a lovley pic of your tank, and just a wooly blur where yur fish are, it is not a technique that has use in an aquarium.