Your water parameters are fine and there is no reason to purchase an ro unit. GREAT NEWS
We probably need a little more info to shed light on the recent death.
How long have you had the rcs? 1 month
Have they been molting normally? yes
Are there fish in the tank? no
Are you running co2? no
Have you recently dosed any products that contain copper? nope
How do the rcs behave? eating moving flying around at times
What have you been feeding them? they dont eat anything i feed them. but it has lots of plants and moss