# hard water to soft in one water change ?



## eminor (4 Nov 2021)

Hello, I collect rainwater because I plan to introduce plants that require soft water, the aquarium actually contains 35 liters of water (51-54 gross). Each week I change 20 liters, last week i changed 20l using 75% tap water and the rest rainwater.

my Kh is 11 and my GH is 17, how high will it be if I use 20 liters of rainwater <= 1 KH, I'm not very good at math

is it risky to go from hard to soft water in one go? thanks


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## MichaelJ (4 Nov 2021)

eminor said:


> Hello, I collect rainwater because I plan to introduce plants that require soft water, the aquarium actually contains 35 liters of water (51-54 gross). Each week I change 20 liters, last week i changed 20l using 75% tap water and the rest rainwater.
> 
> my Kh is 11 and my GH is 17, how high will it be if I use 20 liters of rainwater <= 1 KH, I'm not very good at math


Hi @eminor Assuming the KH and GH of the rainwater is 1 just for the sake of exemplifying the arithmetic.
KH: 35 liters of 11 KH minus the 20 liters you take out, adding in 20 liter of 1 KH water that gives you  (35-20)/35 x 11 + 20/35 x 1 = 5.28 KH
GH: 35 liters of 17 GH minus the 20 liters you take out, adding in 20 liter of 1 GH water that gives you (35-20)/35 x 17 + 20/35 x 1 = 7.86 GH



eminor said:


> is it risky to go from hard to soft water in one go? thanks


I suggest you do it gradually say if you target 8 GH you can do 5L per day over 5 days...

Day 1: (35-5)/35 x 17  = 14.6 GH
Day 2: (35-5)/35 x 14.6  = 12.5 GH
Day 3: (35-5)/35 x 12.5  = 10.7 GH
Day 4: (35-5)/35 x 10.7  = 9.17 GH
Day 5: (35-5)/35 x 9.17  = 7.86 GH

(above assuming your rainwater is GH 0)

Of course after that you need to start mixing in tap-water and/or minerals to maintain your target GH/KH levels. If you do tap-water rainwater mix after that its: 7.86GH / 17GH x 20L = 46% tap water and 54% rainwater (assuming your tap-water is 17 GH and your rainwater is 0 GH). This will maintain your GH at ~8 GH and KH ~5.

Cheers,
Michael


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## eminor (4 Nov 2021)

MichaelJ said:


> Hi @eminor Assuming the KH and GH of the rainwater is 1 just for the sake of exemplifying the arithmetic.
> KH: 35 liters of 11 KH minus the 20 liters you take out, adding in 20 liter of 1 KH water that gives you  (35-20)/35 x 11 + 20/35 x 1 = 5.28 KH
> GH: 35 liters of 17 GH minus the 20 liters you take out, adding in 20 liter of 1 GH water that gives you (35-20)/35 x 17 + 20/35 x 1 = 7.86 GH
> 
> ...


thx so much !!!


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## MichaelJ (4 Nov 2021)

eminor said:


> thx so much !!!


Your welcome.


eminor said:


> I collect rainwater because I plan to introduce plants that require soft water


Keep in mind that a GH of 8 is  not considered to be soft water - its more moderately hard to hard actually.... Its a bit of a vague notion. Soft water is generally considered to be ~4 GH or below. So if you want to go down to 4 GH you need to mix  75% rainwater with 25% tap water (again, assuming your tap is about 17 GH) - this is generally considered too soft for invertebrates - in case you have shrimps, and not many plants require soft water like that, and some won't even thrive in it.

Cheers,
Michael


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## eminor (4 Nov 2021)

MichaelJ said:


> Your welcome.
> 
> Keep in mind that a GH of 8 is  not considered to be soft water - its more moderately hard to hard actually.... Its a bit of a vague notion. Soft water is generally considered to be ~4 GH or below. So if you want go down to 4 GH you need to mix  75% rainwater with 25% tap water - this is generally considered too soft for invertebrates - in case you have shrimps, and not many plants require soft water like that, and some won't even thrive in it.
> 
> ...


thx, i'll try 50/50 at first and see how plants and fish react to it =)


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## eminor (26 Dec 2021)

hello, i did a 90% water change, tds is 120 ppm, let's say i want to change 50% of water, how much water to use to get 120 ppm again, tap water is 240 ppm, rain water is 20 ppm ? thx


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## X3NiTH (26 Dec 2021)

I’m also crap at math but this equation should get you there in your instance.

(TDS1 x V1) + (TDS2 x V2) ÷ (V3) = TDS3

V3 = V1 + V2

V1 = (V3 x TDS3) - (2 x TDS2) ÷ TDS1

V2 = V3 - V1

Put your numbers to that and you get -

V1 = (17.5 x 120) - (2 x 20) ÷ 240 = 8.58

V2 = 17.5 - 8.58 = 8.92

Take 8.58L of 240TDS Tap water and add 8.92L of 20TDS Rain water to achieve 17.5L of 120TDS water.


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## hypnogogia (26 Dec 2021)

eminor said:


> hello, i did a 90% water change, tds is 120 ppm, let's say i want to change 50% of water, how much water to use to get 120 ppm again, tap water is 240 ppm, rain water is 20 ppm ? thx


I’d keep it simple and do a one to one ratio mix, ending up with 130ppm.


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## MichaelJ (26 Dec 2021)

eminor said:


> hello, i did a 90% water change, tds is 120 ppm, let's say i want to change 50% of water, how much water to use to get 120 ppm again, tap water is 240 ppm, rain water is 20 ppm ? thx



Ok, @eminor its time for some algebra 😱.

The total amount of water doesn't matter with the math below - its all ratios.

the formula is:   tap_tds * (tap_ratio) + rain_tds * (1 - tap_ratio) = target_tds

So if you want to target 120 ppm for your water change water and your tap is 240 and your rainwater is 20, it goes like this:

1) 240 * x + 20 * (1-x) = 120

2) 20 + (240 - 20) * x = 120

3) (120 - 20) / (240 - 20) = x

x = ~ 0.4545

Rainwater percentage = (1-x) * 100  ==  54.54%
Tapwater percentage = x * 100  == 45.45 %

240 * (0.4545) + 20 * (1-0.4545) = 120

So if your tank is 35 liter and you change 50% or 17.5L you

add 54.54% rainwater or 9.54 Liter
and add 45.45% tap water or 7.95 Liter

7.95/17.5 * 240 + 9.54/17.5 * 20 = ~120 ppm.

If your adding fertilizer right after WC and want to preserve your low TDS you may want to aim lower, say 80ppm (if your adding say 40ppm of ferts)

Substitute 120 with 80 in step 3)

(80 - 20) / (240 - 20) = x

Rainwater percentage = 72.7 %
Tab water percentage = 27.2 %

240 * (0.272) + 20 * (0.727) = ~80 ppm

Hope this helps,
Michael


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## MichaelJ (26 Dec 2021)

hypnogogia said:


> I’d keep it simple and do a one to one ratio mix, ending up with 130ppm.


Sure, but that will only work for @eminor for as long as the tap and rainwater comes out as  240 and 20ppm.... No harm done knowing how to calculate it. I'd say


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## MichaelJ (26 Dec 2021)

X3NiTH said:


> Take 8.58L of 240TDS Tap water and add 8.92L of 20TDS Rain water to achieve 17.5L of 120TDS water.


Hi @X3NiTH, 

This is off:  
8.58/17.5 * 240 + 8.92/17.5 * 20 = 127.85 ppm.

Cheers,
Michael


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## X3NiTH (27 Dec 2021)

Yeah I did say I was crap at Math, specifically algebra, I’ve been sat here nervously waiting for someone to inevitably correct me! I tried, I get close but no cigar!


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## X3NiTH (27 Dec 2021)

Dunno if I’m climbing deeper into an algebra hole or climbing out of it by fudging my equation to fit your solution but here goes anyway. 

V1 = (V3 x TDS3) ÷ (TDS1 - TDS2)

So

V1 = (17.5 x 120) ÷ (240 - 20) = 9.54

V2 = 17.5 - 9.54 = 7.96

V3 = 17.5L @ TDS120


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## eminor (27 Dec 2021)

amazing, thanks guys, i'm the worst in math in the world actually don't worry =)


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## plantnoobdude (27 Dec 2021)

softwater plants dont care about gh mate, they care about kh only. i'd move the tank over to full rainwater remineralised.

0kh, 6dgh is a good start.
you can use calcium chloride dihydrate (CaClX2H2O), and Magnesium sulphate heptahydrate. (MgSO4X7H2O)
2g of the calcium salt and 2g of the magnesium salt will get roughly 6dgh in 20l water. and it's from 27 Ca and 10ppm Mg.
This is generally enough for most tanks but looks out for calcium or magnesium deficiency. for magnesium it is very crinkly leaves with pronounced veins.
Mg defiency, how much to dose?  see this thread to see what magnesium deficiency looks like.
calcium deficiency looks like older leaves pointing downwards, just general stunting and young leaves coming out small.
In my tank i dose calcium and magnesium at 2:1 ratio 30ppm calcium to 15ppm magnesium because otherwhise i get deficiency.

kh 5 is still too much for softwater plants. i'd use full rainwater.
Honestly what I did when I swapped water from tap to ro is to remove 5l and add 5l of 6dgh water until my gh went all the way down to 6.


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## dw1305 (27 Dec 2021)

Hi all,


MichaelJ said:


> Sure, but that will only work for @eminor for as long as the tap and rainwater comes out as 240 and 20ppm.... No harm done knowing how to calculate it. I'd say


I think it is useful to have a <"ball park guestimation"> for the relative volumes of water, but I'd just use the TDS meter to give you a mix <"within a datum range">. I use rainwater and it varies quite a lot in conductivity through the year, while our tap water has fairly <"consistent parameters">.

cheers Darrel


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