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

[fabsroman]

I'm sitting here about to buy a RO filter because the TDS in my tanks continues to go up. I'm looking at RO filter and RO/DI filters and wondering what the difference is. Can anybody explain this to me, and how the difference will affect my freshwater tanks?

[KarlTh]

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

[Conski]

in freshwater RO is pretty unessential, it means the world to reef tanks but freshies can live without it im sure

[CWO4GUNNER]

Speaking for fresh water they pretty much can live without it, however the concern is rising TDS that accumulate when your tank dehydrates sending RO (actually distilled) water into the air leaving heavy minerals behind and then you replace that lost RO water with more heavy tap water so that your tank continues over time to accumulate dissolved solids until it reaches a critical point displacing oxygen.

Of course this is not really an issue if you have low TDS water (soft). But for someone who has very hard water (TDS 600-800) it can be a real concern. So I have been using RO to replace only evaporated water trying to maintain a TDS below 1000. But becasue of repeated claims that it is not needed I am conducting an experiment in one minor tank where I am only replacing lost water with tap water to see if it is truly detrimental. So far TDS is 1100 and slowly climbing even with water changes.

I can tell you that the pool man who maintains the public pool here told me that pools in the area must be completely drained and scrubbed every year otherwise solids will build until the water turns green and will not longer absorb chlorine or any other pool chemicals making the water smell bad and thick, about 4000 TDS.

[fabsroman]

I just measured the TDS coming out of the tap and it is 185. The TDS coming out of the refrigerator water filter is 120. My 29 gallon tank is at 290, my 20 gallon is at 275, and my 10 gallon is at 260. However, all 3 of those tanks were set up this year, with the two smaller ones being set up just a couple months ago and the 29 gallon being set up in April.

The tanks I am worried about are 75 & 55 gallon tanks that I have at my parents. The TDS in those tanks was around 800 the last time I measured it, but they have been set up for 4+ years at this point. I'm just wondering if my smaller tanks at my place will be in the same place in a year or two, and if it will affect the shrimp that I have in these smaller tanks in the short run. I tried keeping shrimp in the 55 and they were dead within a month.

[CWO4GUNNER]

Well here is the fly in the ointment when it comes to TDS meters. They are calibrated to measure a given assumed substance or Ion by measuring water conductivity usually Cal/Mag hardness. The same stuff the API GH/KH chem test for and why you need to do both test and here is why.

When it comes to your aquarium a TDS meter can give false or exaggerated readings if you have added sodium salt or Potassium salt to the water as these two substances conduct electricity up to 5 times more then Cal/Mag. But you have to know how much sodium or Potassium is in there and why Reef keepers usually test there new water with a TDS meter before adding salt and when they do test their salt water they consider reading based on salt water not fresh water so a saltwater TDS reading of between 5000 and 50,000 is acceptable but would be uninhabitable by any fish if those reading were from freshwater hardness (Cal/Mag).

Example My A.Cichlid community tank has a TDS reading of 1100 and according to the EPA my fish should be dead since oxegen cannot be sustained at minimum levels (5-10PPM) in fresh water with that much hardness (Cal/Mag). However my fish are healthy and lively and my O2 readings are 7.5 PPM and my API GH/KH tester reads a GH PPM of 54 and a KH PPM of 108, how can this be?

Becausethe hard Cal/mag ions in my tap water are run through an ion exchanger (water softener) which exchanges these hard Cal/Mag ions for Potassium Ions or salt depending on what you use (I use potassium) which do not affect O2 levels just like sea salt or sodium ions dont affect 02 levels and becasue sodium and potassium ions conduct electricity 3-5 times more Cal/Mag the TDS meter exaggerates the readings by 3-5 times just like it exaggerates the readings for saltwater.

So make sure for your freahwater tanks you use both a TDS meter and an API GH/KH liquid test kit to get a general idea you actual TDS will be somewhere in between these two test reading, so my Cichlid tank TDS reading of 1100 PPM and GH/KH reading of 54/108 PPM (ignores salt/potassium) is probubly around 1100(TDS)-54(GH)= 1046 divided by/3 (reduce salt ion conductivity by 1/3) = 348 actual TDS plus or minus 10%

[tanker]

If your TDS is getting higher, it is not from the incoming water==unless you are not doing W/C.

[CWO4GUNNER]

If your TDS is getting higher, it is not from the incoming water==unless you are not doing W/C.

The only way you can keep TDS from increasing is to do a 100% water change and sand washing, anything less and old water remaining behind after a water change will continue to collect solids from the evaporation and other sources such as fish waste and food. Even though that increase my be slow without a 100% water change and substrate wash it will and must increase, its a mathematical certainty.

Oh there is one exception however, there are some very expensive particle binders (phosphate removers) on the market that can actually make your filter sticky and collect phosphates and other solid invisible particles and actually reduce overall TDS. for this reason its probubly a good idea to completely clean your aquarium every couple of years depending upon your true TDS which anything over 800 PPM for fresh water is considered uninhabitable for fish according to the EPA.

[fabsroman]

Well here is the fly in the ointment when it comes to TDS meters. They are calibrated to measure a given assumed substance or Ion by measuring water conductivity usually Cal/Mag hardness. The same stuff the API GH/KH chem test for and why you need to do both test and here is why.

When it comes to your aquarium a TDS meter can give false or exaggerated readings if you have added sodium salt or Potassium salt to the water as these two substances conduct electricity up to 5 times more then Cal/Mag. But you have to know how much sodium or Potassium is in there and why Reef keepers usually test there new water with a TDS meter before adding salt and when they do test their salt water they consider reading based on salt water not fresh water so a saltwater TDS reading of between 5000 and 50,000 is acceptable but would be uninhabitable by any fish if those reading were from freshwater hardness (Cal/Mag).

Example My A.Cichlid community tank has a TDS reading of 1100 and according to the EPA my fish should be dead since oxegen cannot be sustained at minimum levels (5-10PPM) in fresh water with that much hardness (Cal/Mag). However my fish are healthy and lively and my O2 readings are 7.5 PPM and my API GH/KH tester reads a GH PPM of 54 and a KH PPM of 108, how can this be?

Becausethe hard Cal/mag ions in my tap water are run through an ion exchanger (water softener) which exchanges these hard Cal/Mag ions for Potassium Ions or salt depending on what you use (I use potassium) which do not affect O2 levels just like sea salt or sodium ions dont affect 02 levels and becasue sodium and potassium ions conduct electricity 3-5 times more Cal/Mag the TDS meter exaggerates the readings by 3-5 times just like it exaggerates the readings for saltwater.

So make sure for your freahwater tanks you use both a TDS meter and an API GH/KH liquid test kit to get a general idea you actual TDS will be somewhere in between these two test reading, so my Cichlid tank TDS reading of 1100 PPM and GH/KH reading of 54/108 PPM (ignores salt/potassium) is probubly around 1100(TDS)-54(GH)= 1046 divided by/3 (reduce salt ion conductivity by 1/3) = 348 actual TDS plus or minus 10%

Well, now I have a headache. My brothers already call me the mad chemist. Can't imagine what they would think if I start trying to figure all that out. Nothing can be simple, can it?

[fabsroman]

The only way you can keep TDS from increasing is to do a 100% water change and sand washing, anything less and old water remaining behind after a water change will continue to collect solids from the evaporation and other sources such as fish waste and food. Even though that increase my be slow without a 100% water change and substrate wash it will and must increase, its a mathematical certainty.

Oh there is one exception however, there are some very expensive particle binders (phosphate removers) on the market that can actually make your filter sticky and collect phosphates and other solid invisible particles and actually reduce overall TDS. for this reason its probubly a good idea to completely clean your aquarium every couple of years depending upon your true TDS which anything over 800 PPM for fresh water is considered uninhabitable for fish according to the EPA.

Now that I agree with. There is no way to bring TDS levels back to what they were when the tank was initially started For instance, tap water like I have at 180 ppm and my 29 gallon tank with a 290 ppm, would ony come down to 235 with a 50% water change. The insane thing is that thi 29 gallon tank was set up in April and I just did a 70% water change and it comes back at 290 ppm.

[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.