Nutrients occur from feeding the fish and water changes. The primary source of "hard" minerals like calcium especially and magnesium is the water change, so the GH of your water is a factor. Floating plants are very fast growing, which means they need lots of "food." Supplementation of nutrients (plants require 17 of these) is usually beneficial, sometimes mandatory, depending upon the light, fish load and plant species and numbers. A comprehensive supplement is the best option for floaters as it ensures everything is available. You aree in WA state, so you should be able to track down Seachem or Brightwell Aquatics, in fish stores or online. Seachem manufacture an excellent comprehensive, Flourish Comprehensive Supplement for the Planted Aquarium which I have been using for over 12 years. Brtightwell Aquatics manufactures FlorinMulti which is much the same. Either of these are your best bet.
As for fish, yes, these things do affect them, more the soft water species that hard water, but provided you do not overdose you are OK. Get the smallest bottle of whichever you select of these two, it will last months as you use so little. Be certain to get the exact same name of either product, as they make several very different (and some downright dangerous) products under these brand names.