pH/KH crash

[dpatte]

Just on a whim i decided to test the pH of two of my established tanks - ive done no major changes to either of these tanks in months.

The first tank had a pH of 7. quite expected.
But the second tank had a pH of <6!!!! (off the scale of my test kit)

As I have no CO2 in either tank, I was shocked by the result. I actually took the test twice because I couldnt believe it.

So i decided to test further.

The first has totally normal chemistry for Ottawa. GH 5 KH 2

But the tank with the low pH has quite unexpected hardness

GH 12 KH 0?  

Any idea whats going on here, and how I can get the tank back to normal numbers? What is the effect of low KH, and how do i restore it?

where did my carbonates go?

[BigDaddy]

Is it a planted tank?  I've read that in a planted tank, in the absense of readily available CO2, that plants will being to strip out their carbon source from the kH.

[dpatte]

Yes, a heavily planted tank (overgrown actually). Besides water changes, what would be the best way to increase the kH, and will this help restore the pH as well?

[Marx]

crush coral in filter.. or oyster shells..

[BigDaddy]

Much easier way is with backing soda.

Add a 1/4 tsp at a time and measure kH and pH changes until you get where you need to be.

Funny though.. if you are doing regular water changes... the kH should have been replenished through the tap water.

Could this be Old Tank Syndrome?

[dpatte]

very perceptive. I was out of town for an extended period and didnt do regular water changes. The plants grew well, but consumed all the carbonates. This just hows how essential regular water changes are.

[artw]

I am sort of more concerned about how come your GH is 10.
...
david do you just top off your tank

[dpatte]

The tank is now fine:

Yesterday 1 changed 1/3 of the water
This brought the PH from 6.0 to 6.4 and the kH from 0 to 1

Then today 1 changed another 1/4 of the water and added 1/2 teaspoon of baking soda.

The pH is now = 7.1  and the kH is 2

Regular water changes should be sufficient now to restore and maintain most of the proper chemistry.

Regular Water Changes: the easiest way to improve your water chemistry