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?
RO vs. RO/DI filter - what is the difference
- Thread starter fabsroman
- Start date
CWO4GUNNER
USN/USCG 1974-2004 Weps
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.
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.
Last edited:
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.
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
USN/USCG 1974-2004 Weps
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%
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
Josh Holloway--Be mine!!!
CWO4GUNNER
USN/USCG 1974-2004 Weps
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.
Last edited:
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?
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.