How often to change filters??

[Torpus]

Just got a new TDS meter today, its a nice inline one. Tests the water going into my RO/DI and the water coming out...
145 going in and 16-19 coming out ... :(
Seems like I change my air water ice filter every six months. Is this normal? I want 0 ppm...what should I do? I've been using the reefkeeper 2, 3 stage with de-ionizer. We've had this for awhile more than 3 years-ish ... I tried searching the model on the site and couldn't find it.
I will change my filters again, but am I getting the wrong ones??
This seems expensive to be spending $100 on filters every six months...
please advice
thanks!!!

[Khandurian]

I was unable to find the model your talking about. Can you specify the layout?

1 prefilter
2 carbon
3 membrane
4 di

Is that what your talking about?

Lets just go ahead and guess thats what your talking about. With that in mind, the only filters your need to replace is the prefilter, carbon and DI. You do not have to replace the membrane every time.

That filter pack would be roughly 25$ In fact, if thats all you need, you could get away with the chloramine filter pack!