# Evaporated water; tap water or osmosis water to refill?



## mede (30 Sep 2013)

I have an open aquarium and now the weather is changing (dry), the water evaporates more quickly then before.

As I have osmosis water available I was wondering if it is better to use osmosis water to refill evaporated water, or can I just use my tap water that I also use during my water changes?

I was just wondering if one has more benefits then the other?


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## Alastair (30 Sep 2013)

Some people will say to add the ro water and some the tap.  Personally I just top up with tap but then I have very soft water from the tap where as others may have very hard so top up with ro

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## Andy D (30 Sep 2013)

Just to add to what Alastair has said in that with soft water topping up from the tap has little impact but with hard water you can significantly increase the TDS so using RO prevents this.


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## Mark Green (30 Sep 2013)

Had the same problem myself over the summer months the tank water was getting close to 32c. On some days I refilled about 1 to 2 ltrs, used normal tap water left standing from the night before. Not sure if that helped much but to be on the safe side I did a partial water change in the week to help with the tds. You could also use a fan as this will help lower the temp of your water.


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## mede (30 Sep 2013)

Thanks all!

I have ro water available, so I'm going to use this for refilling my tank. Interesting to know that it has influence on the TDS.


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## Mark Green (1 Oct 2013)

Ro water will do just fine, wish i had that as an option.


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## Andy D (7 Oct 2013)

mede said:


> Thanks all!
> 
> I have ro water available, so I'm going to use this for refilling my tank. Interesting to know that it has influence on the TDS.


 
Its been a long time since I did chemistry but I belive that when the water evapourates it is just pure water. Therefore it leaves all the chemicals behind. Therefore you have the same amount of chemicals that make up the TDS but in less water. Therefore the concentration (TDS) has increased. RO water with zero or little TDS balances this back out again.

To keep the math simple lets just say you have a tank with a TDS of 200 and half the water has evapourated. The tank TDS is then increased to 400. If you add tap water with a TDS of 200 back to the tank you will then have a TDS of 300 (400+200)/2. So higher than the starting point. If you add Ro water the TDS will revert to 200 (400+0)/2.


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