Yes the kit lens is the 17-85mm. It is an ok lens when bought with the kit. You won't find a Canon lens like that for $100 so it's worth it to buy the kit. I don't know what lens comes with the 20D kit however. I also have the 75-300mm USM, it's nice, but lacks the sharpness at 300mm I would expect from Canon.
The best lens for taking shots of a tank is the Canon 2.8/f 100mm USM macro. It's my favorite lens to use for macros and portraits. The hood is pricey at $35 and really not needed for tank shots, but useful for insects and flowers. The clarity is comparable to L glass for about half the price. I found mine for just over $400 from B&H. One thing to keep in mind with this lens is that you will need a good tripod and don't think about using auto-focus too much, the subject will be gone by the time the camera auto-focuses with this lens. It may be a little different with the 20D though.