To reach a specific concentration of fertilizers, you will need to consider the total water volume (180 gal).
Since the fertilizer dissolves in the tank water, it makes no difference if you dissolve it first in water or not. It will last exactly the same time. Unless I misunderstand your question.
Thanks for the info, I have been going with total, so that good news
I guess the question about mixing in water is should I mix certain amount of water with certain amount of dry fertilizer to get liquid? Then dose liquid? I guess will dry last as long as liquid say if I mix dry with 1000 ml of water, then figure out how much to dose the tank?