I know the difference this isnt my first rodeo..
mg (milligrams) is weight (well, mass actually but that doesn't matter).
mL (millilitres) is volume.
1mL of pure water weighs 1mg, so if the substance or as is watery and not a thick gloopy liquid then it's safe to say that 1mg of it will be equivalent to 1mL, or as near as makes no difference.
But the liquidex bottle is 30ml. 1mg/ml