I don't know what kind of soil nutes that need to be dissolved in hot water you are referring to..except maybe that slow release shit from walmart.
It's always best to use hydro nutes in hydro...as they contain ingredients that would normally be found in soil, but not available in water by itself.
You can get by just fine for awhile with some peters or Mg electric blue lookin nutes you can find at walmart or where ever...but it's best to get some hydro specific nutes eventually.
Drink!