Explanation if dKH test needed

[Llg]

Please don't flame me. I started my tank with tap water as I was going to do a FOWLR tank. However, as most know, once you get started you eventually want corals. I added corals and everything looks good. I have growth on corals, etc. I do 20% water changes weekly, but recently started having some algae growth. I bought a couple of reef kits (I already had the basics for pH, Nitrate, ammonia, etc). My dKH tested at 20 on the Hannah reader and the salifert. I have SPS in the tank and it seems happy. Would there be something in the water that is interfering with the kit, but not bothering the corals?

I am now switching to RoDi today and doing a slow change over; however, this has made me curious. Thanks

 

[blaxsun]

20dKH seems extremely high - what test kit did you use?

 

[Llg]

I used both the Hannah and Salifert tests and both registered high. Corals don't show it

 

[laverda]

I seriously doubt you at 20. I am pretty sure all your corals would be dead at that level. From what i have heard the Hanna Alk kit is not very good. I have never used that one one so i can't say specificly.. I have used most of the other Alk test kits and like the Salifert the best. I suspect you reading you results wrong or a testing error. The current salifert alk test kit does not even give results that high. The highest direct reading it will produce is 15.7 dkh once you dose a full ml of reactant. Review the directions and test again. If you get the same results, I would have someone else test your water.
It is interesting that both test kits gave you the same result.

 

[Randy Holmes-Farley Verified Community Expert]

If you have high alk in your tap water, the new salt water will have very high alk, and high alk does not necessarily cause problems if there are sufficient nutrients available.

Try testing the alk of you tap water to see if that's the cause.

 

[HuduVudu]

@Llg
Just a note LPS IMO don't like alk levels that are this high (nearer 8 is much more ammenable). I would avoid them with alk at this level.

 

[Llg]

I tested the tap water and both tests read too high. I ran a bit of rodi into the tank today, and the Hannah kit is now at 17. You are correct, I used the whole salifert 1 ml and didn't get the color change. I had to another .2 ml to get the change. So I do believe that the Hannah is correct. The duncan in the tank is adding heads and the birdsnet and stylo look great. I think that there has to have been something in the water that was interfering with the reagent. I don't believe it is really that high, but something is causing a misread on both kits.

 

[Randy Holmes-Farley Verified Community Expert]

I tested the tap water and both tests read too high.

Exactly, the tap water is adding alkalinity. Nothing usual needs to be explained if I understood what you did.

Tap water with alk of 10 dKH plus salt mix with alk of 10 dKH gives salt water with alk of 20 dKH.

 

[Llg]

Exactly, the tap water is adding alkalinity. Nothing usual needs to be explained if I understood what you did.

Tap water with alk of 10 dKH plus salt mix with alk of 10 dKH gives salt water with alk of 20 dKH.

Even the tap water with no salt added tests over 20 dkH

 

[Randy Holmes-Farley Verified Community Expert]

Even the tap water with no salt added tests over 20 dkH

some tap water is that high, but perhaps you are using the test incorrectly. nothing interferes

well water or town water?

Your town will produce a water report that shows the alkalinity.

 

[Billldg]

If you have high alk in your tap water, the new salt water will have very high alk, and high alk does not necessarily cause problems if there are sufficient nutrients available.

Try testing the alk of you tap water to see if that's the cause.

Interesting, I never really thought about the actual water source feeding into tested levels. I just always worried about heavy metals and such.

 

[Randy Holmes-Farley Verified Community Expert]

Interesting, I never really thought about the actual water source feeding into tested levels. I just always worried about heavy metals and such.

I show in this article (older now) that some cities have up to 15 dKH in the tap water.

Aquarium Chemistry: Tap Water in Reef Aquaria

As will become clear in this article, using tap water is risky business. Short of testing your own water, there is no way to be certain that your water is acceptable.

reefs.com

 

[Llg]

It is city water. I am just surprised that the corals are healthy and growing in that high dKH. I did a 20% change with rodi and it now reads at 17. Will wait a few days and do another change. Should I change it another 20% or should I go to smaller runs of 10% when dropping the dKH?

 

[Randy Holmes-Farley Verified Community Expert]

I’d likely watch it for a few days and see if it is dropping on its own. It may just be allowing hard corals to grow faster than usual.