In the old days, you could try warming filters. But with the advent of digital, you do everything in the digital domain. You will need to fine tune your Nikon D70's white balance until it is close to the color result you are looking for, then do the final manipulation in post process (with programs like Photoshop). Lens filters are a lot more expensive and difficult to get right.
What Len says. Find a piece of white or 18% gray plastic, stick it in your tank, and do a custom white-balance off of it. I pretty much start every shoot with a gray-card shot.
And shoot RAW...lots easier to fix white balance issues than with a jpg.
if you have a copy of photoshop, go to "new adjustment layer" then "levels" then "auto" and then "OK". This will elimainate your colour cast.
Good luck!
