ok
here are the dials on th canon
A-dep = auto depth of field this means when you take a picture the nearest and Farthest objects will both be in focus
M = manual you can do change all the settings like aperture, shutter speed, and ISO by your self and focus ( you need to read a beginners guide to photograpghy to get this part)
AV=
Aperture priority mode , everything is auto except for the aperture you can set this your self by rotating the dial to change the f stops ( aperture ) the highest numbers means the less light coming in threw the camera and the image will have everything like the foreground and background in focus, the lowest number means more light and this is how you get an object to pop out. The background will be blurry and the object in focus will only be detailed
TV= everything is auto except the shutter speed you can change the shutter speed my rotating the dial. when the fish are swimming fast you need a faster shutter speed to capture the image or it will be blurred. you can play around with the shutter speed to get different effects
P= I forgot what P means LOL I think its ISO mode you can change the ISO ? I pretty sure that might be it, I have taken pictures in a while. Ok I checked it is ISO the higher ISO means you are shooting in a very low light situtsation or you want a grainer look on the image. The lower the ISO mean a finer image and is like daylight or normal lighting sitituation , again you can play with this to see what kind of effects you will get. But since our tanks have MH I think setting it to about 100 would be fine.
The little picture icons
Green rectangle= Automatic ( you don't have to mess with ISO,shutterspeed, or aperture) Your are limited to making special effects ( I use manual all the time) Make sure your lense is set in auto also when using this setting
Face= portrait
mountain= landscape
flower= close up
running man= action shots ( faster shutter speed to catch fast moving objects)
Man with flash= night mode
zig zag arrow with line through it = no flash ( your flash will not pop up)
hope that helps you can find websites that show you pictures of the different settings ( basically same demos you can find in a beginners book)