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Discussion Starter #1
Hi all,

Can someone tell me if I can use potassium phosphate of this kind: K2HPO4.3H20.

What I need to know about this substance is:

1. Is it safe to use in aquaria
2. Is it providing the same amount of potassium and phosphate as the other kind.
3. If someone knows how much P and K will this supply (ppm)

Thanks in advance!
 

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If you are going to use it as your sole soruce of potassium, you are going to put in around five times as much phosphate as the plants need. Your K:pO4 ratio should be roughly 6:1. With phosphate so high, you might have problems with the phosphorous precipitating out with your iron additions or with calcium. It would be better to use it as a source of phosphate and provide the rest of the needed potassium from K2SO4 or KNO3.
 

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Discussion Starter #3
Yes, I am usually using Potassium Nitrate and I am planning to only add a little of the K2HPO4.3H20 to make the P reach somewhere around 0,5-1.00 mg/l.

Nitrate I get from the Potassium Nitarte
Then I add a little Potassium sulphate
For the P I was wondering if I should use K2HPO4.3H20
 

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Yes, KH2PO4 is the usual source of phosphate. You might consider MgSO4 (epsom salt) as a way of adding both magnesium and sulfate. Epsom salts are easy to get, and using KNO3 gives your plants enough potassium, especially if you use it as your sole nitrate source.
 

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Discussion Starter #5
The NO3 is the ONLY source of N I use (I only have two Ottocinclus affinis in my tank-no other fish)

Epsom salt I use and I use the Gh and Ca tests to see how much Mg I have.

Is there a difference between KH2PO4 and K2HPO4
 

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I was referring to the use of KNO3 as the only source of nitrate. You could also use Ca (NO3)2. There is a nutrient mix for hydroponics called Hogland Solution that has equal amounts of Ca(NO3)2 and KNO3. To make Hogland you use 1 molar solutions of KNO3, Ca(NO3)2, MgSO4 and KH2PO4. To make one liter of Hoglands:

5 ml 1M KNO3
5 ml 1M Ca(NO3)2
2 ml 1M MgSO4
1 ml 1M KH2PO4
Add distilled water to bring volume to 1 liter

They use this strength for hydroponics of terrestrial plants. For aquariums, I mix up a Hogland concentrate 5 times as concentrated as above. (I tried a 10X concentrate, but I got a precipitate of calcium phosphate.) Using my 5X concentrate, I add 1 ml per gallon of aquarium water two times a week in thickly planted tanks. Adding at this rate keeps the nitrate level at 1 to 5 ppm depending on how well lit and how thickly planted the tank is. Basically, I monitor nitrate level and add to keep it in the measurable range. I don't worry about measuring other nutrients, and I change water quite infrequently, something like a 50% change every 6 months.

KH2PO4 is preferred over K2HPO4 in Hogland solution because it is a little more acidic and it keeps the pH down around 6 which is preferred.
 

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Yes, I am usually using Potassium Nitrate and I am planning to only add a little of the K2HPO4.3H20 to make the P reach somewhere around 0,5-1.00 mg/l.

Nitrate I get from the Potassium Nitarte
Then I add a little Potassium sulphate
For the P I was wondering if I should use K2HPO4.3H20
Assuming you're adding like 10ppm NO3 from KNO3, K2HPO4 seems a very nice way to get to the rest of your K target without needing K2SO4 or KCl!

** Actually, by mass you'll be adding about a little less K from K2HPO4 as you would PO4. So, if you need a P source, great. Solubility should be the same as the more commonly cited KH2PO4 and its not enough K to matter.
 

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Discussion Starter #8
The K2HPO4 gives me about the same amount of K and P and I aim for around 0.5 mg/l P. Then the KNO3 gives me around 6.5 mg/l K when I am dosing it to reach the N levels of 10 mg/l.

This way the ferts I use for N and P give me around 7 mg/l K and I add even more K from KSO4 to make it reach around 20 mg/l.

The Fertilator helps me A LOT. It is such a nice thing to have and use....
 
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