Your filter looks no different than one of these...
Tap Water Filter (Aquarium Pharmaceuticals)
If so, then you're NOT using RO
water... just DI
. When the cannister is new, you're probably getting pure water but after about 50 gallons it probably isn't doing much. It all depends on the purity of your incoming water. You should probably get a hold of a TDS
meter and check the purity of the output on that thing.
water first passes through a RO
(reverse osmosis) membrane and then a DI
(deionizing) resin. Those two things in parallel give you pure water. If you're using actual RO
water, there would be no reason to put any of the water conditioning additives in it since it would be pure to start with. But if you're NOT using RO
water, then yeah... you should be using some type of dechlorinator at the least.