RO vs. RO/DI filter - what is the difference

[Amphiprion]

Both produce water devoid of its ionic content, but by different means. I doubt it means much to the fish.

Actually, most reverse osmosis membranes are fairly inefficient at reducing many common (and small) ions. That is why deionization resins are needed if you want to reduce ion content to lower levels. Certain ions, like H2S, silicate, phosphate, and CO2 aren't entirely removed and may not be removed satisfactorily for at least some applications. Granted, small, uncharged molecules can and will pass quite freely through the entire unit, regardless of the inclusion of DI resins. Keep in mind that this applies to TDS as well, since the real TDS is at least a little higher than what you are measuring.

 

[CWO4GUNNER]

You know when I said water changes cannot lower TDS I was talking about within given set of parameters. In other words using the same TDS source tap water, and not intervening with RO or DI which would certainly bring down TDS, another mathematical certainty.
It would be interesting though to see how much TDS would drop, significantly Im sure, if an 800 TDS tank was 50% water changed with DI water. Probubly less then 50% due to the sand and other ornaments coated with TDS that would be reabsorbed by the DI/RO water.

 

[Amphiprion]

I knew what you meant.

Yep. All the higher TDS water that is trapped in various interstices that would slowly diffuse over time. You could chart the rise, even in the absence of any extra input--i.e. if you took all TDS-creating factors out.

 

[gmh]

I would just get the straight RO system if possible. It would be simpler and cheaper to maintain. The membrane will remove most dissolved solids, like 95% or more I believe. That should more than suffice. The DI system removes almost everything else but do you really need virtually distilled quality water?

 

[fabsroman]

I would just get the straight RO system if possible. It would be simpler and cheaper to maintain. The membrane will remove most dissolved solids, like 95% or more I believe. That should more than suffice. The DI system removes almost everything else but do you really need virtually distilled quality water?

I am wondering the same thing, especially after reading the reply right before you where the DI system will remove phosphate and CO2. All of my tanks are pretty well planted and I run CO2 to them, and sometimes I add phosphate. I really don't think I need to remove those out of the water that I am going to put into the tank after a water change.

I'm going to heed your advice and get the RO filter and leave the DI filter for another day should I really find that the RO filter isn't enough.

 

[jpappy789]

I would just get the straight RO system if possible. It would be simpler and cheaper to maintain. The membrane will remove most dissolved solids, like 95% or more I believe. That should more than suffice. The DI system removes almost everything else but do you really need virtually distilled quality water?

:iagree:

 

[Amphiprion]

I agree with just an RO unit being necessary for freshwater systems. I was just pointing out that membrane filtration can leave a lot more behind than one might think.

 

[dundadundun]

this is a great discussion imo. i've seen a lot of people suggesting just doing w/c's with tap water in many cases i thought more info could be relevant. i have watched and tested as my kh/gh rise with additions at every waterchange. then my mathematical/analytical side kicked in and i started topping off with stripped water before doing w/c's. it helps but from time to time i still have to do w/c's with more pure water to keep things where i want them.

this is also one more reason i think suggesting the api master test kit should be outlawed to anyone who may want to actually understand their water chemistry. it is insufficient and lacks too much imho.

 

[Amphiprion]

I guess the salties take the whole TDS thing for granted. We start from square one with every top-off and water change, minus what the inhabitants have created.

 

[jpappy789]

this is a great discussion imo. i've seen a lot of people suggesting just doing w/c's with tap water in many cases i thought more info could be relevant. i have watched and tested as my kh/gh rise with additions at every waterchange. then my mathematical/analytical side kicked in and i started topping off with stripped water before doing w/c's. it helps but from time to time i still have to do w/c's with more pure water to keep things where i want them.

this is also one more reason i think suggesting the api master test kit should be outlawed to anyone who may want to actually understand their water chemistry. it is insufficient and lacks too much imho.

I've yet to have problems with tap water residing in two places with reportedly high TDS in the municipal water. Maybe I'm just lucky.

As for the API tests, they're great for nitrogen testing...not to expensive and relatively common. I wish they would include GH/KH as those can usually give you an indication of where your TDS lie.