The trick to shooting through a fish tank is understanding the concept of White Balance. Although it is a simple concept, I find it hard to explain myself so here are some info off the internet:
http://www.dpreview.com/learn/?/Glos...Balance_01.htm
http://www.photoxels.com/tutorial_white-balance.html
http://www.cambridgeincolour.com/tut...te-balance.htm
So what you should do is set something white inside the fish tank and fill up the frame with it. Then manually set your white balance so that the camera adjusts it meter accurately. Recompose the scene without the white object and shoot. This should give you a more accurate color reproduction then using AWB. A caution for using a white reflective object is that it may cause your cameras exposure meter to under expose the scene, making it look darker then it is. To compensate for this set your manual exposure control to 1 or 2 stops ( depending on the white object ) to overexposing the scene.
Another way to control White Balance is to shoot in RAW. Shooting in RAW, your camera will not preselect a specific White Balance for you, but rather shoots the scene as it sees it. Using an editing software such as Nikon Capture you can easily correct the photo to the appropriate White Balance setting. Editing with a RAW software is a lot easier then having to mess around with Levels and Curves in Photoshop.
Another tip is to shooting close up macros with an aperature of around f/8 or f/11. Using f/8 or f/11 the whole macro object will be relatively sharp but you might need a tripod as the shutter exposure can get low resulting in a blurred image.
If you try shooting with an aperature value of f/2.8, you can increase your shutter speed, therefore you can hand hold your camera; however the focus distance is very shallow and you may only have a sliver of the macro image in focus. Luckily this is a digital world and it is cheap to erase and try again.
Because you are shooting through water and water magnifies, you need to shoot straight on perpendicular to the object or else you will get a horrible magnifying effect. Make sure that your lens is positioned as close to to object as possible to help reduce magnification. You can also move the object closer to the front of the fish tank so that you are not shooting through much water. Hope some of these tips will help.
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Suzuki RV125