This is my humble attempt at providing some guidance on photography with canon dslr
First of all, before you start any phototaking, off all your wavemakers. As in our tank, there are very fine particles flowing around and those particles will appear on your photo, and it will distract the viewers. If your tank have 2 pumps, 1 for turnover and the other for chiller, turn off the turnover pump as well. Then you can start cleaning the tank of any algae and let the particles settle down for around 30min or so.
secondly, get those glass cleaner and start to spray it on newspaper and start wiping the exterior tank glass. You wouldnt want to take photo with a glass that is full of stain as it would make the photo turn out to be blur and not as sharp.
now preparation is all done. take out your dslr and start to adjust the setting. Aperature should be set high, range of f5.6 to f11 would be sweet. Shuttle speed should alway maintain it at 1/50 or faster. set your metering mode to spot metering, this would have your object be nicely exposed. for photography of fish or any moving object, set your focus mode to al servo. As for iso, it really depend on your lighting, but try to keep it as low as possible without compensating your aperature or shutter speed.
next, i would give you a tip on white balance. As our tank will be under the same lighting unless you change your bulb. So set your dslr to shoot raw and take some photo. afterwhich open your canon processing programme and start to change the white balance under the custom setting. Amend it until your photo show the same color as what you see in real life, then from that few photo, take the most commonly used setting and set your dslr custom white balance to be of the same value, so from now onward, always use the custom white balance when you are shooting your tank.
lastly before taking any photo, always make sure your camera is parallel to your tank glass as it would reduce the distortion caused by the glass. Enjoy shooting and do post some of the photo taken using the guide as above.
If you have any suggestion to make photoshooting easier and nicer, do pm me and i will add it to this guide