Tufa rock and KH, peoples experience.

maff2026 · Feb 27, 2026

35 gallon tank
Ph 8.3
Calcium 400
Magnsesium 1300
Salinity 1025
Kh 13.
Rocks, 60% tufa, 40% coral rock.
Substrate arragonite.
Instant ocean salt.
Is it the tufa thats buffering my kh?
Even after water changes it still goes back to 13.

Randy Holmes-Farley · Feb 27, 2026

Have you tried that alk test on the new salt water?

maff2026 · Feb 27, 2026

Randy Holmes-Farley said:
Have you tried that alk test on the new salt water?

I can't get my head around this. I've just made some fresh RODI water at 0.00TDS. made a mix with instant ocean at 1024 and the KH is off the scale. Either the instant ocean is laced with KH or the test kit is useless.
If i measure pure RODI water with no salt for KH it shows 0, nothing.

Randy Holmes-Farley · Feb 27, 2026

maff2026 said:
I can't get my head around this. I've just made some fresh RODI water at 0.00TDS. made a mix with instant ocean at 1024 and the KH is off the scale. Either the instant ocean is laced with KH or the test kit is useless.
If i measure pure RODI water with no salt for KH it shows 0, nothing.

IO has pretty high alk. How are you measuring alk?

maff2026 · Feb 27, 2026

Randy Holmes-Farley said:
IO has pretty high alk. How are you measuring alk?

I'm in UK, using a British made KH test kit called NT Labs, a refractometer and hydrometer that both agree with each other after calibrating the refractometer.
Hydrometer and refractometer both agree that RO water is 1000 and salinity is at 1024 where I set it.
The stupid thing is, this test kit is measuring my tap water at 15 KH before salt has even gone in when it is known that my tap water is about 4KH.
If I mix tap water with salt at 1024 SG then the KH goes off the scale at 18KH.
It goes to 15KH when I mix RO water with instant ocean when it should read below 10.
I think i'm going to get the salifert to clarify things.

Randy Holmes-Farley · Feb 27, 2026

Yes, sounds like time for a new kit. :)

Not sure on the below 10 dKH, however. At least in the US, above 11 dKH when mixed with RO/DI is not unusual. That's what rtparty got from testing by ATI.

maff2026 · Feb 27, 2026

Randy Holmes-Farley said:
Yes, sounds like time for a new kit. :)

Not sure on the below 10 dKH, however. At least in the US, above 11 dKH when mixed with RO/DI is not unusual. That's what rtparty got from testing by ATI.

My friend tested our tap water to be 4KH with a salifert. If I mix our tap water with instant ocean it should be about 12 max and no more. It takes two full 100ml sringes for a yellow reading which is 18-20KH. Something ain't right. If I mix RO water with instant ocean at 1024 SG, I shouldn't be getting more than 9 max but again its almost double that at 1.6 shringes before going yellow. A reading of 9 is about 90ml of test fluid, this takes 160ml before it turns yellow which is way off.

JTP424 · Feb 27, 2026

Interested if confirmed the Tufa rock is safe?

Randy Holmes-Farley · Feb 27, 2026

maff2026 said:
My friend tested our tap water to be 4KH with a salifert. If I mix our tap water with instant ocean it should be about 12 max and no more. It takes two full 100ml sringes for a yellow reading which is 18-20KH. Something ain't right. If I mix RO water with instant ocean at 1024 SG, I shouldn't be getting more than 9 max but again its almost double that at 1.6 shringes before going yellow. A reading of 9 is about 90ml of test fluid, this takes 160ml before it turns yellow which is way off.

Why do you think IO is only adding 8 dKH? That is not what we see in the US.

Randy Holmes-Farley · Feb 27, 2026

JTP424 said:
Interested if confirmed the Tufa rock is safe?

Tufa is not necessarily a single thing that is always the same. Folks have asked over the years and some have used it. I cannot really say with certainty that any given batch of stuff is ok from the standpoint of trace elements and such.

maff2026 · Feb 27, 2026

Randy Holmes-Farley said:
Tufa is not necessarily a single thing that is always the same. Folks have asked over the years and some have used it. I cannot really say with certainty that any given batch of stuff is ok from the standpoint of trace elements and such.

Hi Randy. Its not the tufa, I'm getting the same results with instant ocean and RO water in a fresh mix out of the tank. Even if I were getting 11KH this test kit is not showing no where near that, its showing over 15KH. Its showing tap water and instant ocean between 18-20KH when we know my tap water is 4KH.
So unless IO can raise tap water by 16 points then the test kit is garbage and I've just been reading others are having trouble too.

maff2026 · Feb 27, 2026

I've used NT Labs for donkeys years but I'm afraid its time to cut loose.
One guy has reported the latest kits to be 3 points out compared to Salifert. His KH is 9 on Salifert and 12 on NT Labs. Mine is showing 15 but I think it will be actually closer to 10 or 11, a full 4 points out.
Why do I think this? Because high KH causes the calcium to drop out rapidly with precipitation. I have no precipitation at all and my calcium is a stable 430, not indicative of very high KH.