Lets keep this between us its my little secrate on how to get great photo's. I'll give you the non Tripod rendishion since im not sure you have one.
1) Take a white tile as big as you have one and putt it in the tank. If your shooting a paticualer area ad it there for most accurate but it doesnt make a big deffrence.
2) Bump your ISO up to 400 so you can get hand held settings
3) This is where it gets fuzzy since i dont have your exact model but press the WB button on the back. Then go to custom you might need to scroll over.
4) Now once you have clicked on Custom under WB you will go to the "regular" back screen
5) now simply aim and zoom in to a focused picture on that white tile ( BTW that tile needs to be new fresh and white)
6) after you take that picture it Automaticly setts your white balance to a accurate, doing that basicly says " hey camera what i jusyt shot is what white realy is and negates all the extra "blue" in the tank