I am wondering why you think your tank needs more iron than that which the CSM provides. Do you have a lot of red-leafed plants? If not, remember that iron is a trace or micro mineral and is only needed in very small quantities. If you are putting your CSM+B into solution, only mix as much as your tank will use in a 2-3 week period and keep it stored in a cool, dark place like a refrigerator. This reduces the growth of any microorganisms that have found their way into the solution and want to consume the nutrients there.
I use Seachem and wonder if they add something to keep it fresh? An iron preservative you might say. Seems to store for months well. I also notice that compared to iron sold at garden centers,Seachem is much more concentrated..double at least.