trace element top-up question....

[Dez]

I was wondering. I have an 18 gallon reservoir for water top-up in my 90 gallon tank. It's gravity fed with a float valve. I want to put some of my trace elements like iodine, vitamins and stuff in my reservoir so I'm not putting it in manually weekly into my sump. Is there a formula/equation or easy way of figuring out how much of the stuff to put into the reservoir so that I get the right amount of "stuff" in my main tank? I probably have to fill my reservoir up every 3 weeks to a month.. My gut instinc tells me just to put about 3 weeks worth of vitamins and iodine in every time I fill my reservoir...does that make sense?