To be honest...I haven't taken any tank shots since I got the 20d, my tank crashed last summer after days without power from the hurricaines and I have not restocked.
The sigma is a decent lens...but at 300mm your going to need higher shutterspeeds to hand hold, non-moving objecs that won't be a problem, but it also has a 3ft minimum focusing distance for macro. It will do decent for you, you might look into the canon 50mm 1.8 for a good all purpose (cheap) lens, I know several people that do fish with that lens. You should look into a good macro lens that will give you 1:1 magnification, the sigma at 300mm is only 1:2, the canon 100mm macro is very popular and with some close up diopters like the canon 250d or the canon 500d you can get very close and great magnification.
Good luck and you got yourself a nice camera
As for online sites...take a look in my sig