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post #1 of 3 (permalink) Old 05-29-2015, 01:29 AM Thread Starter
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ro or no

my tank is getting alot of water evaporation. should i set up a RO system for top offs or no? its a freshwater tank and i do weekly or bi-weekly water changes. This is for my angel tank. the pH of the tap water is around 7.4


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post #2 of 3 (permalink) Old 05-29-2015, 03:10 AM
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PH is pretty much irrelevant here...

What's your GH like? Are you already uncomfortable with how hard it is, or is it ok?

As your water evaporates, the calcium and magnesium stay behind, increasing the GH of the water.

Topping off with RO will bring you exactly back to where you started.

Topping off with tap water will also reduce GH from where it was after evaporation, but will only bring you halfway back from there to where you started.

So, if mild (percentage wise) increases in GH are OK by you, use tap water.

If the idea of increasing GH at all bothers you, then top off with RO, and do it often.

Personally, I top off with tap, but I have a glass lid, so I'm really only topping off a little bit each week.. maybe 5% or so (about 1.5 gallons out of 36). So, when I'm about to top off later in the week, my GH is up 5%, after adding tap, I'm up by 2.5%.

New to planted tanks, avid gardener/tinkerer.

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post #3 of 3 (permalink) Old 05-29-2015, 04:54 PM
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If you want the actual math for the impact of replacing evaporation with tap instead of RODI, assume:

T = tap water TDS (or other linear measure)
E = percent evaporated
C = percent changed (of total, so includes make-up tap)

Between each water change the water will reach lower steady state of

Lower = T * C * (1-E) / (C - E)

And the maximum prior to top off will be

Lower = Lower / (1-E)

If you crank through those with some typical values, a 50% water change with 10% water loss to evaporation, starting with 200ppm of dissolved solids, will alternate between 225 and 250ppm.

On the other hand less water change of 20% with the same will alternate between 360 and 400 (double the original) in steady state (taking almost a year to reach it with weekly changes).

Reduce the evaporation to 5% with 20% changes and the same 200 TDS and it alternates between 253 and 266ppm.

This presumes (and given this is "planted tank" it is likely not true) that the dissolved solids are not consumed by plants, and also that other sources (rocks) are not contributing new solids.

Linwood

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