I personally use the little test strips for chlorine. Any hint of color and it gets changed. The big blue carbon is the way to go IMO. If you have the space. Much longer contact time so it lasts MUCH longer. I was burning through regular carbons. Now I’m 10k+ through the big blue and still going.
the other thing you need to keep in mind is water pressure. Need a minimum of 40psi at the RO membrane. The lower the pressure the less effective the membrane operates. I have a booster and keep my pressure at the membrane right at 80psi.
the more canisters and filters you put BEFORE the membrane the lower your pressure is going to be at the membrane.
i put a pressure gauge on each side of my sediment filter. When I see a 10% drop in pressure, I change the sediment.
you really only need a single .5 micron sediment then one ore two carbon then your RO then a couple DI.
I have .5 sediment -> big blue -> 2x 90gpd RO -> high capacity DI -> silica buster DI.
I have low TDS source water but high sediment. And also my water has the dreaded chloramine
You’ll see I have two empty canisters. These were previously my sediment and carbon. But empty now because of big blue.