Calculating correct dosing for RODI water treatment? - The Planted Tank Forum
 
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post #1 of 6 (permalink) Old 12-17-2015, 08:53 PM Thread Starter
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Calculating correct dosing for RODI water treatment?

Hey everyone!

I've been trying to figure out how to best treat my RODI water after a WC, as well as the balance of fertilizers to use in my tank. I used to use Seachem for water treatments and fertilizing but it's just getting to be too expensive and I really want to be in control of exactly what is going into my tank.

Some general info about my tank:
29 gallon tall
Eheim 2215 filter
Fugeray Ray 2 & Fugeray Planted+
Pressurized CO2 (regulated with pH controller)

I'm planning on dosing using the EI method 3x weekly. Using the Fertilator, I've figured out how much KNO3, KH2PO4, and K2SO4 I need to dose to get the parameters as specified by the Fertilator. I'll be using Plantex CSM+B for micro nutirients.

Now, I'm kind of stumped with the RODI water treatment before it enters the tank. What GH/kH range should I be shooting for? I'm planning on using MgSO4 and CaCl2 to get the GH up, and KHCO3 to get the kH up, only I don't really know how to figure out how much of each of these three to add to my water. I saw here (Help with calculating correct dosage of cacl2 and mgso4 - General Aquarium Plants Discussions - Aquatic Plant Central) how to figure out the GH and am pretty sure I understand it. But I'm pretty lost in terms of what to do regarding the kH.
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post #2 of 6 (permalink) Old 12-18-2015, 12:04 AM
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TL;DR Add 3.89279 g of KHCO3 to 29 US gallons to raise kH by 1 degree. Caveats, see below.

0. Someone check my math please. I did this right after dinner and without coffee. The idiot factor may be in play.

1. This depends on what you mean by kH. If you mean TOTAL alkalinity (anything that will buffer H+), then other anions including the amount of KH2PO4 from your EI dosing may need to be considered. If you mean CARBONATE alkalinity, then the math is easier and only has to account for the KHCO3 but may not represent the actual buffering capability of the tank.

2. The amount of KHCO3 will need to be adjusted for the ACTUAL volume of the tanks. 29 gallons minus substrate, plants, filters, etc.

3. The amount of K in the EI dosing will need to be adjusted to account for the potassium in KHCO3.
______________________________________________
Number crunching

Molar mass
potassium bicarbonate = 100.11514 g/mol
calcium carbonate = 100.0869 g/mol
carbonate = 60.0089 g/mol
bicarbonate = 61.01684 g/mol

Assuming carbonate alkalinity, 1 German degree = 17.848046 mg/L CaCO3.
In water, CaCO3 dissociates into one calcium ion and carbonate ion.

So, to determine how much carbonate is required to shift kH by 1 German degree
1 dkH = [(X * 60.0089) * (100.0869 / 60.0089)] / 17.848046
X = 0.178325 mmol/L of carbonate

To convert mmol/L of carbonate to mg/L of bicarbonate
(1 carbonate binds 2 H+ whereas 1 bicarbonate only binds 1 H+)
0.178325 = (X / 61.01684) / 2
X = 21.7221 mg/L of bicarbonate

To back calculate the concentration of potassium bicarbonate
(21.7221 / X) = (61.01684 / 100.11514)
X = 35.4609 mg/L of potassium bicarbonate

To calculate how many grams of potassium bicarbonate to add to 29 US gallons of water
(1 US gallon = 3.78541 liters)
X = (35.4609 * 3.78541 * 29) / 1000
X = 3.89279 g of KHCO3
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post #3 of 6 (permalink) Old 12-18-2015, 03:17 AM
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Quote:
Originally Posted by CourtneyLeigh View Post
What GH/kH range should I be shooting for?
This mostly depends not on plants (majority of plants grow well in very broad range) but on your fish requirements.
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post #4 of 6 (permalink) Old 12-18-2015, 01:14 PM Thread Starter
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kH differences

Quote:
Originally Posted by Zhylis View Post
This depends on what you mean by kH. If you mean TOTAL alkalinity (anything that will buffer H+), then other anions including the amount of KH2PO4 from your EI dosing may need to be considered. If you mean CARBONATE alkalinity, then the math is easier and only has to account for the KHCO3 but may not represent the actual buffering capability of the tank.
If you don't mind, could you explain the difference between the total alkalinity and carbonate alkalinity? My biggest concern with the kH is figuring out what pH to set the controller to so there's enough CO2 in the tank, but not enough to gas the fish. From my understanding, too low of a kH can result in pH crashes which I definitely do not want.
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post #5 of 6 (permalink) Old 12-18-2015, 01:27 PM
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Total alkalinity is how well the water resists changes in pH. This buffering can come from many different salts and other materials. The major thing in fresh water tanks that does this is bicarbonate.

Carbonate hardness is a measure of carbonates and bicarbonates.

It is my understanding that most hobby level KH tests are actually measuring how well the water resists a change in pH. (Alkalinity).
Since the most common buffer is the carbonate/bicarbonate/CO2 reaction, it is reasonable to call these tests KH tests.

I would do this:
Make up whatever amount of water you want to use for a water change.
Add about 1/2 of the minerals etc. you think are right.
Circulate the water for at least half an hour, but longer is just fine, to make sure they are all dissolved.
Test. The results should be almost exactly half the values you are aiming for.
If not, then do some math and add the remaining minerals. Circulate, test.

If the end result is still too soft, allow the water to circulate longer, perhaps overnight, and test again. Then add more minerals if needed.
If the end result is too hard, then add some water to dilute it.

No matter how carefully you do the math, it will always come down to actually making the recipe and testing it.
Once you have figured it out you will not have to wait until all the minerals are dissolved before adding the new water to the tank. I just set up the new water first, and run a small pump on it while I am doing the water change, cleaning the filter and so on. By the time I am ready to use the new water to refill the tank the minerals are well enough dissolved.
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post #6 of 6 (permalink) Old 12-18-2015, 02:04 PM Thread Starter
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Originally Posted by Oso Polar View Post
This mostly depends not on plants (majority of plants grow well in very broad range) but on your fish requirements.
My tank has 15ish tetras, one rainbowfish, 6 cory cats and one clown pleco (used to have a ram pair but they recently passed, can't complain though, they were alive and kicking for almost 3 1/2 years). I think I'm going to shoot for a dkH of 4 for now, that seems to be an acceptable value for most tropical fish
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carbonate hardness or kh, estimative index, fertilizer, german hardness or gh, water treatment

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