Planted Tank Guru
Join Date: Jan 2010
Location: Contra Costa CA
Use a minimum of 2 containers. Macros in one, micros in the other.
Specifically, do not mix phosphate with iron. Other than that make as strong or as weak a solution as you want, as suits the volume the pump will dose out. At some fairly strong solution no more fertilizer will dissolve, so if you are stirring a lot and cannot dissolve any more that is too strong a mix.
Light can deactivate some chelated fertilizers. I would keep the mix in a dark container, and cover it.
Air is probably not itself a problem, but an open container can accumulate dust, people may throw things into it, danger to children... As much as possible close the container. Highly likely you are depending on air entering the container so the pump can do its job, but plenty of air can enter through a very small opening, you sure do not need a wide open bucket for this.