I know there are some point and shoot cameras probably in that price range that can do a decent job, but I can only speak to SLRs, so I'll limit my advice to that. I'd recommend looking for a used body as was previously suggested. I'm a Canon guy, so considering your price range, I'd probably look for a used 20D or 30D. I say used, because you could end up spending pretty much your entire budget on my next recommendation. . .the lens. My bread and butter for macro aquarium photography is the Canon EF 100mm/f2.8 macro. Looking at Adorama, it's selling for about $530 new. The 50mm Canon macro is about half the price, but I really like the extra reach of the 100mm. So, seeing as you're not going to find even a used SLR for $70, what are your options:
1. Sigma sells a 105mm macro for around $480. . .still probably not a viable option if you're trying to keep the package under $600. Between a used lens and a used body, you may be able to do it, though.
2. Look into extension tubes. They're a cheap way to turn a normal lens into a macro lens. I'm afraid I've never used them before, but I'm sure others can give you advice here if that's something you might be interested. Otherwise, look in the macro section here, and you should be able to find plenty of info:
http://photography-on-the.net/forum/. The other benefit to this is if you decide to invest in an actual macro lens later, the original lens you bought is still a perfectly good lens for your other photos.
3. Go with a 50mm macro lens. I don't think it's ideal, especially for larger aquariums, but it's still going to be better than probably any point and shoot.
4. Scrap the whole SLR idea and look into a good quality point and shoot camera. Again, not an area where I can help, but it's certainly an option.
And if all that's not enough, factor a tripod into the equation. You can get away with a cheaper tripod, but if you want a quality tripod, you'll have to be prepared to spend at least a couple hundred on it. On the other hand, if you're just looking at using it to snap the occasional aquarium photo, dealing with a $50 tripod is probably worth it.