You can use any brand of filter that will fit your unit.
I would stick a TDS
meter in it and see what it is... it might be fine to use in your FW
tank depending on the rejection rate of your system and the hardness of the water going in. Many people collect the water and use it to water plants. I have even seen a couple of folks put the waste water line into the washing machine and use it to wash clothes (fill the washer with it instead of tap water). You could also mix it with tap water if you want to use it in the FW
tank... that would reduce the TDS
Bottom line, Unless the water going in is really, really bad, it would probably be fine for your FW
tank. Most RODI
systems produce 1 gallon of RODI
water and waste 3 or 4 gallons, depending on how many membranes you have and the temperature of the water. That means at worst the waste water will be 25% higher in TDS
than your normal tap.