TDS when mixing RO & TAP water - The Planted Tank Forum
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post #1 of 5 (permalink) Old 08-26-2016, 09:03 PM Thread Starter
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TDS when mixing RO & TAP water

When I mix 1/4 RO and 3/4 TAP to soften my water could that cause TDS to be too low? I'm confused how this works.

My RO water pH is 6ppm, 0 dKH, 0 dGH. (Is that normal? It sounds like distilled water... Which is a no-no, right?)
My TAP water pH is 8.0, 6 dKH, 6 dGH.

My aquarium pH is 7.8, 2 dKH, 4 dGH.

The reason I mix them is to decrease the stress on my betta and invertebrates after a water change. They show signs of stress if I change 25-40% with only TAP water because the KH and GH immediately increase 2-3 degrees.

Considering my RO water parameters are almost nonexistent, if I mix 1/4 RO and 3/4 TAP to get target 4 dGH and 2 dKH could this deprive the aquarium of necessary TDS?

Please help me understand this lol!
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post #2 of 5 (permalink) Old 08-27-2016, 04:24 AM
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When adding R/O water to Tap water, I would be more worried about my KH than the TDS

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post #3 of 5 (permalink) Old 08-27-2016, 07:34 AM
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There are some inverts that like 6 dGH, so lowering GH isn't necessarily a good thing, unless they receive all they need from diet.

TDS is total dissolved solids. It measures minerals, salts, waste, etc. Basically, everything in water that is organic or inorganic in the water column.

RO water should have between 5-7 pH, and 0 on everything else, except maybe TDS. It can show low TDS but still be RO. Low as in, preferably under 15 TDS. You might say that RO water is distilled water, but distilled water isn't always RO water.

TDS is less important than GH and KH, in the sense that it's possible to have high TDS and low GH or KH, or "low" TDS but higher KH and GH. TDS more or less measures how "clean" the water is. The lower the TDS, the cleaner the water is. The higher it is, the dirtier the water is. (it also measures fertilizers, too...) With inverts, at least shrimp, there is a target range of TDS to go for, although that would depend on the type of shrimp you have.
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post #4 of 5 (permalink) Old 08-27-2016, 07:53 AM
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I mix the two also.

TDS tap: 30
TDS di/roi: 0

TDS result in tank: 225-250

GH: 6-10
KH: 2-3
PH: 7.4

Depends a lot on what is in the tank...
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post #5 of 5 (permalink) Old 08-27-2016, 07:21 PM Thread Starter
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Thank you for the explanations.

I'm not trying to lower the GH and KH in my tank. I'm trying to soften the replacement water to match the tank water. My substrate UP Aqua Sand has a high CEC and buffers the water. It's a mixture of clay and soil, not real sand. I've been told not to change more than 15% for shrimps. However, that wasn't keeping my tank clean enough. I figured if I can soften the replacement water closer to the tank parameters I could change more water at once, 25-40%.
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