A Soluble, Safe, and Effective Buffer

[Bill]

Hey All!

 

I've been doing a fishless cycle for about 3 weeks so far, but I've noticed that my tap water is not buffered. My pH is pretty stable ~7.8 or so--originally it was around 7.2 but after adding ammonia to start/maintain the cycle it tends to go up (ammonium hydroxide is basic). I presume that once the cycle is well established and fish are added the pH will decrease, so I just want to be prepared with an effective and GRAS buffer. Here are the three main buffers I've been considering and the various pros and cons that I see with each. If you could please answer any of the questions I have regarding them and tell me your experience using any of them, as I'd greatly appreciate it.

Calcium Carbonate:

Not significantly soluble in water so I've noticed that at a pH around 7.2, it doesn't dissolve and increase the KH (carbonate hardness/buffering capacity) so it will just sit on the bottom of the tank and has a tendency to cloud-up the water and therefore potentially cause issues with filter, pump, and plumbing.

I've heard that it can be self-regulating so that at a lower pH it will disolve and increase the pH back up until the buffering compounds equiliberate. However, I've been testing the solubilty of pure CaCO3 powder (food grade from NOW vitamins CaCO3 Product) and notice that even at a low pH (6 and lower) the CaCO3 does not dissolve into solution, at least not after a day or two--and so the pH and KH stay exactly the same. Can someone explain how long it usually takes for CaCO3 to dissolve into an acidic solution so that it can effectively increase the KH and pH? I've read that magnesium in the water may prevent the precipitation of CaCO3 in solution so possibly if I add magnesium it may assist the dissolution of CaCO3?

Also, how much CaCO3 should you add per gallon if your current pH is ~6.5 and you wish to bring it up to 7.4-7.6?

Also, it is generally regarded as safe, and will provide Ca2+ for the plants and fish.

 

Potassium Bicarbonate:

Readily soluble in water so the carbonate/bicarbonate ions will dissociate immediately and will begin buffering the water as soon as it is added. Because it dissolves there is less clouding (if any) and plumbing issues. However, it is not self regulating so once all of the carbonate and bicarbonate ions are bound, its buffering capacity will cease and more KHCO3 will eventually need to be added. Also, because it is so soluble, overdosing may be an issue (concentrations greater than .5% can be toxic to plants).

It too is generally regarded as safe, and will provide Ca2+ for plants and fish. As you all probably know it is also a fungicide, which is an added bonus.

How much KHCO3 should be added per gallon if your current pH is ~6.5 and you wish to bring it up to 7.4-7.6?

 

Potassium Phosphate Salts:

I don't hear too much about these salts used as buffers, but they contain two primary plant macronutrients: Potassium and Phosphorus. I don't know where these salts (mono-, di-, and tri-potassium phosphates) are sold in the crystalline form, however I have had luck creating them by adding phosphoric acid to potassium hydroxide:

H3PO4 + KOH --> H2O + KH2PO4

KH2PO4 + KOH --> H2O + K2HPO4

K2HPO4 + KOH --> H2O + K3PO4

 

The buffering capacity from mixing those two products seem to yield very strong buffers with a dKH of 12+. However a few days after I added the salt solution to my 100 gallon tank the buffering capacity did diminish, so I don't know if that is a trend with these specific buffering salts or if it was an issue of concentration, purity, or reaction with other compounds in solution. It is something that I would like to look into seeing how both nutrients are taken up by plants and would be beneficial in numerous ways.

If anyone has had any experience using these salts please let me know how your progress has been.

How much of these salts should be added per gallon if your current pH is ~6.5 and you wish to bring it up to 7.4-7.6? 

 

Sorry for the long post. I appreciate any feedback.

 

 

[TCLynx]

I wouldn't use the Potassium Phosphate Salts: since once you are feeding fish, there is usually plenty of phosphorus available from the fish feed.

 

When buffering the pH of a system back up normally people will alternate between a Calcium and a Potassium buffer since using one or the other all the time can cause an imbalance.

 

It would be really hard to tell you exactly how much of something you need to add because water chemistry is really much more complex than all that and you also shouldn't (at least once you are cycled and have fish) move you pH more than say 0.2 in a day so you will need to start testing small doses of stuff if you use any of the fast acting materials (like calcium hydroxide or potassium hydroxide) to see how much it takes you to move your pH about .2 and then you would only add that much per day until you have shifted back up to your desired pH.

 

The slower acting buffers are even trickier.  Now I will often hang a stocking of shells or shell grit (calcium carbonate) in my fish tank and if the pH goes too high, I pull it out.  Now I wish I had something similar for potassium bicarbonate for a slow acting buffer to alternate with the shell stocking.  But in my one system I'll simply add a spoon full of potassium bicarb to each grow bed under the water inlet whenever the pH drops near 6.  I try to keep the pH in that system up over 6.5 and I have to test the pH on that system every few days.

[Bill]

Thanks for the quick reply. I've heard that CaCO3 will bring the pH up to around 7.4-7.6 and then will not bring it up any further. What pH do you find the potassium bicarbonate raises your system to? I'm assuming that because it is so soluble that it will keep raising the pH as you add it, but by how many pH increments do you find one teaspoon raises your system to? Thanks!

TCLynx said:

I wouldn't use the Potassium Phosphate Salts: since once you are feeding fish, there is usually plenty of phosphorus available from the fish feed.

 

When buffering the pH of a system back up normally people will alternate between a Calcium and a Potassium buffer since using one or the other all the time can cause an imbalance.

 

It would be really hard to tell you exactly how much of something you need to add because water chemistry is really much more complex than all that and you also shouldn't (at least once you are cycled and have fish) move you pH more than say 0.2 in a day so you will need to start testing small doses of stuff if you use any of the fast acting materials (like calcium hydroxide or potassium hydroxide) to see how much it takes you to move your pH about .2 and then you would only add that much per day until you have shifted back up to your desired pH.

 

The slower acting buffers are even trickier.  Now I will often hang a stocking of shells or shell grit (calcium carbonate) in my fish tank and if the pH goes too high, I pull it out.  Now I wish I had something similar for potassium bicarbonate for a slow acting buffer to alternate with the shell stocking.  But in my one system I'll simply add a spoon full of potassium bicarb to each grow bed under the water inlet whenever the pH drops near 6.  I try to keep the pH in that system up over 6.5 and I have to test the pH on that system every few days.

[TCLynx]

My 300 gallon fish tank with 6, 100 gallon grow beds system gets about 6 heaping teaspoons whenever the pH drops below 6.2 Plus I have a small bag of shell grit hanging in the fish tank.  If the pH doesn't come up enough for my linking in a couple days, I'll add well water which is full of calcium carbonate.