Simuation with system of micromolar concentration

GROMACS version: 2018.6
GROMACS modification: No
Hello All,
I need to simulate a system with very low concentration. Therefore I have tried simulate 10,000 water molecules within 30^3 nm^3 simulation box. But when I changed the dimension of the box by editconf and simulate 100^3 nm^3 waterbox it shows error like this:

So, as a beginner I would like to ask two questions:
(1) Is this problem with the box length or number of molecules?
(2) What would be standard procedure if I want to simulate system with micromolar concentration?
Thank You. I will appreciate any help.