Many people love the 18-55, for the price and quality it has received many good reviews. It really is a great lens for all around stuff like shooting your tank and family stuff. Pairing it with a flash will be the real thing to help improve your image quality for tank shots. Personally, i am not sophisticated enough to see that big a difference in the image quality, especially not if you just want to share pics online. Many people have also told me that the image quality at f8 is just as good as much more expensive lenses.
As stated above the 35 and 50 1.8g lenses are awesome. However, I would recommend starting with the 18-55 like you have been seeing on e-bay. That way you can get a good feel of what focal length you like shootings with and can pick up one of those nice primes if you start getting a little more serious.