the xt and xti come with different lenses just same distance, but anyway, you shouldnt have to purchase a lens if you are taking pictures of inside your tank, yes inside is dim lighting, but inside a tank should not need a flash, just expose for the light in the tank, really easy to do... learn your in camera meter at the tank in a darker part of the tank, then move your dial so the dot is dead center... this wont give you the best exposure but it will allow you to not have to buy a flash, there are also things you can buy to help get you the right exposure, like a light meter, bout $150-200, or a gray card (gives you a 18% gray reading)... i know it sounds confusing but if read up on metering and zone system you will understand.... i dont like flash with my camera bc i have just the flash on the camera and i have to hold a white card to reflect light, if not it reflects off the glass