Maxicrop with iron could take care of trace element and iron supplementation without having to buy a separate chelated iron supplement. Iron is one of those things that if you source water doesn't provide it, most fish food probably won't provide enough.
I've added greensand to my grow beds before as well but I have not yet gotten away from needing to supplement some iron. Our well water has plenty of calcium carbonate but seems to have no iron. We get hard water stains in the house but not rust stains.
Hi Ricky. No, Maxicrop shouldn't harm the critters, but it will darken your water for a few weeks so you won't be able to see them. Before adding in Maxicrop you might also want to consider a longer term solution of just adding more fish to your system. I'm not a big fan of supplementing a fully cycled system unless it is absolutely necessary, like to buffer pH, which you clearly don't need to do. Feels like treating the symptoms rather than the underlying problem to me. Since your nitrates are at zero you may just not have enough fish giving off enough food for your plants, which would show up as the symptoms you are seeing.
Maxicrop has been safe for the fish I have so far used it with. Just don't use much. Your little system a few drops a week might be appropriate if you don't want to deal with dark water or a cap full for the first dose if you don't mind brown water for a while. I've known people who used seasol (another brand of seaweed extract) heavily for their first year and then found they no longer needed it in later years once the mineralization of the solids in the grow beds really got mature.
One point about aquarium fish foods, many of them are designed to keep the fish happy yet provide the minimum possible amount of nutrients to the water in the aquarium since in a non-aquaponic or planted aquarium, more nutrients means more water changes and more work. So, to get more nutrients from feeding the fish, you might want a fish food that pollutes your water a little more. I don't have much experience with aquarium fish feeds so I'm not sure what varieties I would recommend to get more nutrients to your plants.
if you find any aquarium feeds that seem to supply the needed things better, please share as I know others are interested in this.