As mentioned before for outdoors shoting under the sun a Circular polarizer (good Quality one) is a must.
for tank shooting and any other stuation a UV filter (also good quality) is also a good idea. Scratching a filter is always cheaper than scratching a lens.
Good quality ones come treated and do not cause additional flaring....
Coutrary to popular belief a filter (good quality ones would cost you around 50 usd or plus depending on thread size) is more expensive than a lens. a pro lens could cost you thousands of doolars that's true however that covers multiple glass elements, housing, focusing and metering mechanisms and electronics, Vibration reduction. add to that costs of research behind a lens, testing, and much more complex manufacturing. When you pay 70-80 USD for a high quality UV filter you are paying more money. When I dropped my 18-200 VR and it broke open the UV filter in front of it got scratched badly. the lens had to be sent to Japan to be fixed and costed around 600 uds including shipping (at the time it was selling for 1 K plus in addition to 2 months waiting list) so I had no choice but to fix it. If I had damaged the front element which was covered by the filter that bill would have been much bigger I'm sure.