What is TDS really? Is 0 TDS water necessary?

[TheEngineer]

I know TDS is total dissolved solids and the recommendation is to only use 0 TDS water on your tank, but I don't really know what TDS is and if I need it to be 0 in my tank.

My tank is just zoanthids right now and I don't plan on keeping SPS. Zoanthids tend to like dirtier water (nitrates, phosphates), right? I've always tended to think I needed to keep the TDS at 0 to prevent build up of nitrates and phosphates. If that's true (is it?) would skipping my DI stage and putting 1-2 TDS water into my tank be a smart(er) move?

 

[twilliard]

This is a good one for @AZDesertRat

 

[Nano sapiens]

TDS means any type of dissolved solid (calcium, phosphate, iron, etc.). The idea is to get your water as close to 'pure' as possible so that you are not adding unknown substances. While '0' TDS is the most pure that we can measure, there can still be trace amounts of certain substances (silicates, for example).

Best practice is to use '0' TDS water for any reef tank. However, some people have had success with using water that is not quite as pure.

 

[Randy Holmes-Farley]

TDS is a unit of measure of conductivity.

In seawater, it is several tends of thousands of ppm, because every ion in the water (sodium, chloride, calcium, etc.) contributes to the conductivity.

What we use it for is to measure the effectiveness of an RO/DI system.

The conductivity of top off water need not be zero ppm TDS, depending on what it is in the water that contributes to the TDS. If it is sodium, chloride, calcium, magnesium, etc., then it could be 500 ppm TDS and be perfectly fine.

The usefulness of TDS is that it measures when the DI part of the RO/DI is being depleted (and also monitoring the RO membrane rejection rate). If the DI is becoming depleted and the TDS begins to rise, undesirable ions may be getting through along with those that are OK. So copper, lead, phosphate, nitrate, silicate, ammonia, etc. can all be getting through.

Consequently, if the TDS is not 0 ppm from an RO/DI, it is not clear whether the water is adequastely pure for a reef tank or not (without extensive chemical analysis).

Consequently, aiming for 0 ppm TDs is always desirable.

 

[Randy Holmes-Farley]

FWIW, here's my recommendation of how to use TDS with an RO/DI (tips 8 and 9):

Reverse Osmosis/Deionization Systems to Purify Tap Water for Reef Aquaria by Randy Holmes-Farley - Reefkeeping.com
http://reefkeeping.com/issues/2005-05/rhf/index.htm

from it:

Tips on Using RO/DI

1. If you have a pressure gauge on the RO membrane, change the sediment filter when the pressure drops to about 40 PSI. If the pressure does not rise adequately after replacing the sediment filter, change the carbon filter.
   
   
2. If you do not have a pressure gauge, change the sediment and carbon filters when the output of purified water becomes significantly reduced.
   
   
3. Avoid running the RO/DI if your town’s water system is flushing fire hydrants in your area, as the sediment and carbon filters will clog very fast.
   
   
4. If the sediment and carbon filters do not seem to get clogged, at least replace the carbon filter periodically to ensure that chlorine and chloramine are being broken down appropriately. Both of these can be measured with an inexpensive chlorine test kit if you want to be sure they are being removed.
   
   
5. If you have especially hard water, you might consider running it through a water softener before the RO/DI filter. That process removes calcium and magnesium by swapping sodium for them. While aquarists are not generally worried about calcium and magnesium in their source water, they can foul RO membranes by forming calcium and magnesium carbonate precipitates inside it. That precipitation will reduce flow and eventually make the membrane unusable.
   
   
6. Make sure that your waterline pressure is appropriately high (at least 40-60 PSI). In some situations where the pressure is low, an inline pump may be desirable before the RO/DI system to boost the pressure.
   
   
7. If there is excessive carbon dioxide in the source water (such as well water), you might consider degassing the water first to remove some of the carbon dioxide, and thereby reduce the rate of depletion of the DI resins. Note, however, that this option is not inexpensive, and typically involves repressurizing the water with a pump.
   
   
8. If you are evaluating an existing RO membrane and can collect water from the tap and after the RO membrane, the conductivity (in mS/cm or ppm TDS) should drop by a factor of more than 10 across it (to as much as 100), relative to the tap’s water. If the drop is less than a factor of 10, it is not working properly, and may have holes in it.
   
   
9. Monitor the DI resins by measuring the effluent’s conductivity, either with an inline meter (set to its most sensitive level), or by measuring the effluent manually. If you are using a TDS or conductivity meter, then the measured value should drop to near zero, or maybe 0-1 ppm TDS or 0-1 mS/cm. Higher values indicate that something is not functioning properly, or that the DI resin is becoming saturated and needs replacement. That does not necessarily mean, however, that 2 ppm TDS water is not OK to use. But beware that the flow of impurities and the conductivity may begin to rise fairly sharply when the resin becomes saturated. Do not agonize over 1 ppm versus zero ppm. While pure water has a TDS well below 1 ppm, uncertainties from carbon dioxide in the air (which gets into the water and ionizes to provide some conductivity; about 0.7 mS/cm for saturation with normal levels of CO2, possibly higher indoors) and the conductivity/TDS meter itself may yield results of 1 or 2 ppm even from totally pure water by not being exactly zeroed properly. Also note that the first impurities to leave the DI resin as it becomes saturated may be things that you are particularly concerned with (such as ammonia if your water supply uses chloramine or silica if there is a lot in the source water).
   
   
10. If you recharge your DI resins yourself, be very careful with the acid and base used, as they can be dangerous.
    

 

[Randy Holmes-Farley]

To answer the title question "What is TDS really?"

it turns out I have an article with that exact title which will detail it as much as you could want:

What is TDS? by Randy Holmes-Farley - Reefkeeping.com
http://www.reefkeeping.com/issues/2004-04/rhf/feature/index.htm

 

[TheEngineer]

Thanks, Randy. You always provide such thorough and well thought out responses. I certainly have lead in my water, so don't want that in there. I thought the RO membrane takes that out though. Can't recall what else is in there. I'll continue using RO/DI not just RO (my RO water comes out at 1-2TDS on my inline meter). As I was babying my degassing system it occurred me that perhaps I didn't need the purest water in my tank.

(I'm on a well btw)

 

[Randy Holmes-Farley]

Thanks, Randy. You always provide such thorough and well thought out responses. I certainly have lead in my water, so don't want that in there. I thought the RO membrane takes that out though. Can't recall what else is in there. I'll continue using RO/DI not just RO (my RO water comes out at 1-2TDS on my inline meter). As I was babying my degassing system it occurred me that perhaps I didn't need the purest water in my tank.

You're welcome and thanks.

The RO membrane will typically take out 90-99% of most ions, and in many cases may be adequate. The DI just ensures it is adequate,a nd also takes Out a few which are fairly poorly rejected by the RO, such as silicate and ammonia.

 

[Markonagle]

Hello, My name is Mark and I to questioned if RO water was needed in my system. I’ve been in the Reef tank hobby for about twenty five years now my first reef tank was a sixty six gallon mixed reef with just a media filter and a protein skimmer. I have my own well with a alkaline reading of 8.0 right from the tap. I just used tap water and read the corrals regularly and they looked
Fantastic with lots of coralline algae to. My next system was a built in 180 gallon with all the bells and whistles.
I had a metal halide fluorescent mixed light fixture a sump packed with filtration and nothing but RO water went into the system. After all the expenses and work to have such a system my 66 gallon performance was actually better. Go figure

 

[Randy Holmes-Farley]

Hello, My name is Mark and I to questioned if RO water was needed in my system. I’ve been in the Reef tank hobby for about twenty five years now my first reef tank was a sixty six gallon mixed reef with just a media filter and a protein skimmer. I have my own well with a alkaline reading of 8.0 right from the tap. I just used tap water and read the corrals regularly and they looked
Fantastic with lots of coralline algae to. My next system was a built in 180 gallon with all the bells and whistles.
I had a metal halide fluorescent mixed light fixture a sump packed with filtration and nothing but RO water went into the system. After all the expenses and work to have such a system my 66 gallon performance was actually better. Go figure

Welcome to Reef2Reef!

Tap and well water can be fine, but for some people it is not, having too much copper (usually from your own pipes) or other minerals.