I disagree with Chris on the canisters needing to be clear.
I have the white canisters on my RO/DI. I do have a
TDS meter that I periodically use to test the water coming out of the DI filter. When it goes over 002 TDS I change all my filters (not the RO membrane).
The last stage on the unit in the picture is a carbon filter, it is not needed as Chris said it is purely for taste purposes. Leaving it in will also cause your TDS reading to be high. The water coming out of the DI cartridge should be 000 TDS.
IMHO
Mike