It depends on your expectations. The 75-300 really isn't a horrible lens. I used to have the older model (70-300), and as long as you understand it's limitations, and you aren't shooting for Sports Illustrated, it's a very viable lens. This lens is fairly sharp out to 200mm, and even out to 300, I'd say it's acceptably sharp if you stop down f/7.1 or smaller (smaller aperture that is. . .larger f-number).
For the aquarium, I'd much prefer a 100mm (or thereabouts) macro lens for fish or coral pictures or a wide angle lens for full tank shots.
So, in short, the 70-200 is a great lens, outclasses the 75-300 in every way, and I understand why everyone here is recommending it. That said, I'd say the 75-300 is priced appropriately for what you're getting, and although it isn't professional quality, it's certainly better than an equivalent zoom on pretty much any point and shoot camera. So, if that's all you're looking for, I'd say go ahead and get it. You can save yourself even more money if you find it used (probably from someone who has upgraded to the 70-200L).