# RO vs RO/DI uint



## Dmaaaaax (Nov 20, 2008)

I was looking at possibly adding an RO or RO/DI uint to my sink for my freshwater setup. There is pretty big difference in prices between RO and RO/DI uints. What exactly does the DI portion get rid of that the RO portion does not? What are the benefits and draw backs of having one over the other.

From what I have read, if I go RO/DI then I have at least 1 extra filter that needs to be changed out, and I believe I need a TDS reader to know when the DI filter needs to be changed, so additional cost is one factor.

I have Equilibrium (GH booster) on stock, other than that and pH buffers, is there anything else I need?


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## Dmaaaaax (Nov 20, 2008)

Wow...nobody? SW people can probably chime in also, but I didn't want to double post in 2 areas.


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## phil_pl (Apr 10, 2009)

I am still researching these units, i wasnt ready to buy one but won it in a raffle so im on the fast tract to learn now, i will let you know what i find


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## Oldman (May 25, 2009)

It is pretty simple. The RO will take the water's mineral content to somewhere between 5 and 10% of the original water's mineral content. The DI removes the little ionic compounds that are left behind by the RO. If you are doing a typical freshwater setup, you will need to add minerals back to the water before putting any fish into that water. The typical practice is to mix your RO with some tap water to get the desired mineral concentration. I somehow can't imagine removing every trace of mineral content and then adding a little more back in to get the same final concentration. I believe that salty people are trying to somehow remove every bit of mineral content because they want a clean slate when they add in the artificial ocean in the form of sea salts.


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## Dmaaaaax (Nov 20, 2008)

Does anyone know what the TDS meter is needed for: RO or DI?

Which one gets rid of phophates (not a mineral)?

Does either get rid of silicates?

I have Equilibrium from Seachem to add minerals back in. There is no way in halibut I'm going to add tap back in to my RO water...especially if I don't know exactly what minerals are in there and what are not and in what concentrations.


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## Oldman (May 25, 2009)

Phosphorus and silica are both removed by either RO or DI to a significant extent. I use my TDS meter on my RO water, on rain water, on my tank water and on my tap water. It tells me the TDS of each water so that I can judge the concentration of minerals. The TDS would be essential for evaluating DI because it could detect the tiny traces of minerals that can be present when the DI is not working right. There are silicates in almost any natural water on the planet so if you must avoid them, you will need a DI and will need to change resin in the resin column long before most people would. The first chemical that breaks through a DI is silica as SiO2. When I used large commercial demineralizers to make demineralized water, we would monitor silica to avoid ever seeing a TDS breakthrough. It took 4 chemicals and a spectrophotometer to measure silica at a level that would give us that warning though. Phosphorus is not as common as silica in water but is very common in fertilizer preparations. It is the P in the N-P-K number scheme on common fertilizer products. 
Why the extreme aversion to these particular chemicals?


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## MediaHound (Jul 19, 2006)

Dual stage TDS meter is what I use. I check water going in, and water coming out. 
When the TDS starts to rise, thats time to change the DI resin. TDS is typically 0 coming out.


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## Dmaaaaax (Nov 20, 2008)

Thank you for the great answers.

I have tested my tap for phosphates and it is in the range of 1-2ppm. The target dose for a planted tank is 0.1ppm. Too much can lead to certain algae blooms which is what I am seeing right now. The phosphates seem to be higher in the summer which is probably due to fertilizer run off?

Silicates are not as big of a deal to me but I am noticing some brown algae which usually does not show up in well lit established tanks, so I am assuming that my tap is high in silicates which is brown algae's food source of choice.


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## Oldman (May 25, 2009)

Any glass tank will have some silica in it. The main ingredient in glass is silica type sand.


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