one with **Zinc** 15% EDTA chelated

Welcome to the forum.

It is best to make a few assumptions.

You bag of Zn EDTA contains 15% by weight of zinc. ie 100g of complex contains 15g of Zn and 85g of other stuff. (EDTA chelator and sodium).

That is 15g of Zn in 100g 1.5g of Zn in 10g 0.15g of Zn in 1g and so on.

1 gram is 1000mg so 1g or 1000mg of complex will contain 150mg of Zn.

I mg of X per litre of water is 1ppm of X. As you will be requiring less than 1ppm of zinc in your tank it now becomes a matter of what you can accurately weigh and what volume you want to use.

As an example I weigh about 19.5g of a mix of FeEDTA and an Fe & trace EDTA mix. This is diluted to 5litres to make a stock solution (A). I then take 500ml of stock solution A and dilute to 5litres to make stock solution (B). This way my errors in measuring become vanishingly small. I then take the appropriate volume of Stock Solution B and add that to my tank to get the required amount of Fe and trace elements. Remember most of us make a guestimate of the net volume of our tanks. I choose to use dilute solutions and add 'large' volumes to my tank. Commercially it is the other way round.

It is really a matter of ratios. There is no need to do molecular calculations your reagents that you are using will be re bagged and labelled industrial fertilizers sold as the amount of 'active' ingredient it contains. Think of a gold ring its the % of gold that matters not what the other metals are.

I hope that the above will be of some help and good luck with your endeavors.