Definitely DSLR. With macro lens. However it's not budget solution at all. If you are tight on money, try to put your hands on some Christmas promotion for the body with kit and later add macro lens.
There is no real other choice if you want good quality pictures.
However, as i said - it's expensive hobby, you should already knew it when you start the aquarium. :lol:
P.S.
If i look for cheap solution, i would go for second hand Canon 350D and suitable macro lens (50, 60 or even 100mm macro by Canon). Another solution is Sigma 17-70. You will need one that can do 2.8 aperture.