Hey Bev, in addition to adding the big three I also dose a small amount of strontium. I only dose what is recommended on the jar but not as often, so overdosing isn't an issue. In your case weekly water changes should be more than enough to maintain your trace levels though.
Here is a link to the Seachem site. Even if you don't use Seachem products, I found that the information they provide on their products helpful to understanding the uses for them.
http://www.seachem.com/products/reef.html