knowing the fish's behaviour is very important too. depending whether you want a full side-on profile (like what I want) or you want them in various antic poses, you need to know where a fish is likely to be in the pose you want and wait accordingly.
As for the shoot at an angle thing, if you are flashing over the water, then it is not too important, you seldom get glass reflection, if the fish is not too near the front glass. shooting at an angle introduces distortion by the glass itself. For my needs it was not acceptable as I wish to see every minute details. if you are getting too much front glass reflection and HAVE TO shoot at an angle, get a photo tank with the thinnest glass you can find.
end of the day, after having all the necesary equipment, you need to have patience and practice. but you can get amazing shots from even digicams like the G5. Here is one taken with my G5 with +9 close-up. note this fish is less than 1.5cm long and transparent. the second pix is a 100% crop.
of course using the a DSLR with proper macro lens would be much easier, but your patience and practice will serve you well. A DSLR mainly gives you better image quality. here is the female of the same fish, you can even see the eggs in the belly (taken with 20D and EF 100mm Macro)