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Low KH? Crushed coral or Alkaline Buffer - how to use it?

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im having KH/PH issues—- I’m not a newbie, but new house with new tap water is blowing my mind!!! 

I have very low KH from my tap but very hard water (hardest readable on test strips)and PH at about 7.0 depending on the day.

I’ve been getting PH swings throughout the day that I’ve been tracking for 3 weeks. My ph is swinging from 6.5 to 7.6 throughout the day most days (starting 3 days after water change) 

Yikes! I’m thinking I must need more KH- but how can I get it? water changes are only lasting me about 1-2 days, then I’m readying nearly 0 KH in my tank. So I guess a water change won’t be my best bet. Crushed Coral? Seachem’s Equilibrium?  

I have 1 lb of crushed coral currently mixed into my gravel, but should I use more? No room in my filter to add it, unless I toss one of my 2 sponges out (AC50)  

this is happening in my 29 gallon. 

any help would be greatly appreciated! 

Edited by Ariel S
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Crushed coral would be a good solution, especially if it is in the filter. I have found that more impactful then just using it as gravel.

I also believe Equilibrium/Replenish would be mainly for GH, and have less of an impact on KH. You can also use some baking soda to raise KH/PH, but do it very carefully, and test the measures in a 5 gallon bucket first. I have similar soft/low KH water to you right out of the tap, and 1/2 teaspoon of baking soda per 5 gallons will bring my PH from 6.8 to 7.6, and raise KH from 40 to about 80.

Your mileage may vary, which is why it is super important to test the results outside of the tank first.

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Crushed coral is the best long term, slow and steady method of raising KH. There are all kinds of ways to deploy it. You can fill an old-school box filter with crushed coral and run an airline to it and it'll work a bit faster than mixing it in the gravel and give you more control without costing you space in your existing filter, though it does take up space in the tank. You can also fill an external breeder box (the Marina/Fluval ones for example) with crushed coral and hook up the airline to them and slowly circulate tank water through the crushed coral. The breeder box can hang on the back of your tank and be largely unnoticeable. You don't need one of the big ones for this purpose. If money is no object, you could plumb in a reactor (more commonly used in marine tanks) and fill it with crushed coral.  Things like Wonder Shell and the cuttlebones sold for pet birds can also be used. 

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18 minutes ago, Ariel S said:

Can you explain the breeder box/airline idea to me? 

Sure. The Fluval/Marina breeder boxes use an airlift to move water from the tank to the breeder box and then it flows through the breeder box before overflowing back into the tank. You could simply fill the breeder box with some crushed coral and the water would flow over/through it and then back out into the tank picking up the calcium along the way. The flow rate is adjustable also using a valve that's included with the boxes. 

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When you say your hardness is off the charts I assume you mean gH right? If that is the case you may have a hard time getting your coral (calcium carbonate) to actually dissolve. I would highly recommend dosing your tap water with carbonate salts. Seachem(r) sells a variety for the reef. I thought there was another company that sold this in bulk for ponds but I cannot find it on the googles.


As suggested above I would recommend playing scientist not in the display tank but in a separate container so you can get your dosing right. It doesn't effect fish quite as much as pH swings do, but it's always best not to disturb them.


Reference page for those who want a rundown of water chemistry:


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