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Evaporating water..
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ozziegt
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Joined: 07 Mar 2005

PostPosted: 2005.04.21(Thu)10:10    Post subject: Reply with quote

benedictj wrote:
ozziegt wrote:
When you do water changes you are going to take out the minerals that have built up in the tank due to evaporation as well.


Sort of, but if you don't remove the buildup on the sides of the tank the minerals will dilute, causing your hardness to increase over time. Similarly, if the you allow lots of water to evap without replacement, despite diluting mineral concentrations with water changes, eventually it will catch up with you (excluding the dilution of the 'tank side mineral deposits' if you will).

As always, diligence is the best approach with keeping chem stable. (just some thoughts, sorry for the ramble).


Yeah, but I would imagine you would reach an equilibrium (the amount of minerals added due to evaporation and build-up on the side is the same as the amount removed due to diluation through WCs). Of course the best thing is to clean the sides of the tank regularly or make sure things are topped off properly. I would rather do this than let vinegar get near my tank...
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benedictj
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Joined: 05 Dec 2004
Location: new york, ny

PostPosted: 2005.04.21(Thu)12:17    Post subject: Reply with quote

ozziegt wrote:

Yeah, but I would imagine you would reach an equilibrium (the amount of minerals added due to evaporation and build-up on the side is the same as the amount removed due to diluation through WCs). Of course the best thing is to clean the sides of the tank regularly or make sure things are topped off properly. I would rather do this than let vinegar get near my tank...


Right, of course keeping on top of it is the best bet.

But, what I was trying to say, and may have admittedly bumbled, is that it actually won't reach an equilibrium even if you discount the dilution of buildup. Rather, your hardness would perpetually buildup overtime, even if you still removed the mineral deposits on the side of the tank.

Say for example your tap water has a TDS of 10 mg/l. You lose a 2 litres from your 40 litre tank to evap every two weeks and have a resultant TDS of approx 11 mg/l at the end of your first two weeks. You remove 18l of water and replace it with 20l to fill the tank. Subsequently, the TDS of the tank would be 10.5 mg/l. (This is by taking the mg/l of remaining tank water (11mg/l x20l) adding it to the mg/l of the 20l you replaced via change (10 mg/l)(two combined =420 mg) then dividing that by the total tank volume (420/40=10.5 mg/l ). In this case, devoid of such factors as solid breakdown due to bioload etc., your overall TDS would increase by 1 mg/l every month...

Obviously, this is all theoretical, and the numbers above may not apptly reflect reality, but it's just to demonstrate what I was saying. Eitherway, moral of the story...untended evaporation is bad, very very bad.
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uLtRa
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Joined: 11 Apr 2003
Location: Southend, UK

PostPosted: 2005.04.22(Fri)3:47    Post subject: Reply with quote

What I meant which is completely logical is that if you only water change once every 2 weeks say then a lot of water will be evaporated say 2-3 inches and there will be a lot of build up. Now say that the frequency of water changes is everyday, the amount evaporated will be in the region of millimetres and there will hardly any build up - so saving on cleaning up large amount of cleaning it off. Clearer?
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Oscar916
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Joined: 15 Mar 2005

PostPosted: 2005.04.22(Fri)20:16    Post subject: Reply with quote

My tank water evaporates about 2" every week no matter what I do.. Even when I first started my tank, and I placed 100% new water.. there was still evaporations after about 3 weeks.. So what I do is I just add more water, I usually don't remove any. I just fill it to the top. And I vaccum the gravel every month or month and a half with a 50% water change.
The water is just very hard. I used to try softening the water with the water softner pillow in the past but it never worked..even the pH reducers would work the first 2 or 3 days and then by the 4th day, the water would be back to a high pH and hard water. I haven't found any solution so far that helped.
And some of the white calcium/mineral deposits on my tank hood just seem permanent! I tried everything you could possibly think of to try and get rid of them, nothing works!!
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ozziegt
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Joined: 07 Mar 2005

PostPosted: 2005.04.23(Sat)7:44    Post subject: Reply with quote

benedictj wrote:

Say for example your tap water has a TDS of 10 mg/l. You lose a 2 litres from your 40 litre tank to evap every two weeks and have a resultant TDS of approx 11 mg/l at the end of your first two weeks. You remove 18l of water and replace it with 20l to fill the tank. Subsequently, the TDS of the tank would be 10.5 mg/l. (This is by taking the mg/l of remaining tank water (11mg/l x20l) adding it to the mg/l of the 20l you replaced via change (10 mg/l)(two combined =420 mg) then dividing that by the total tank volume (420/40=10.5 mg/l ). In this case, devoid of such factors as solid breakdown due to bioload etc., your overall TDS would increase by 1 mg/l every month...

This is where I think you might be missing a key factor. While the amount of TDS added is a constant value, the amount removed is a percentage of the total.

Lets take your example:

1st water change before/after: 11/10.5
2nd water change before/after: 11.5/10.75
3rd water change before/after: 11.75/10.875
4th water change before/after: 11.875/10.9375

You see an equilibrium being reached now? The amount after the water change won't pass 11, and the amount before can't pass 12. Because it will be cut in half every time.
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benedictj
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Joined: 05 Dec 2004
Location: new york, ny

PostPosted: 2005.04.23(Sat)8:59    Post subject: Reply with quote

Uncle

You're right, I think (don't have time to actually sit down and do the extended math). Thank god I'm not a chemist- though I always managed to get good marks in it somehow.

Good conversation, IMO.
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Jake
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