You get what you pay for. The sigma 70-300 for $120 will give you very little use. I've used this lens before thinking that you couldn't go wrong for that price but you can. Focusing is ridiculously slow and noisy and at the 300mm range you defiantly need a tripod. For a macro that range is huge and you'll never get a sharp picture out of it, not to mention your minimum focusing distance is something like 36". You really need a prime lens for macro photography.
For a zoom lens the Nikon AF-S 70-300 VR ED is very nice and a good buy at $500. For a macro you'll have to spend a little more but you'll want a prime lens, the Nikon 105 VR is nice but pricey. There are alternatives but macro lenses are a little more of a specialty and you pay for that.
For the aquarium hobby a nice 60mm prime macro lens or something similar would be a good choice, they sell for a reasonable price and he'll get lots of use out of it.
