Rising KH?

[Lukara]

Hi. It has been just about a week now that I've added more light to my tank and in conjunction with that I've begun to dose with my PMDD.

This past Sunday, I did a 50% water change. I tested my KH (using Hagen test kit) 12 hours after doing the water change and it was at 90ppm. The next day I added added my PMDD: 1/2 tsp of K2SO4, 4 drops of phosphate (from Fleet Enema), an additional 2 ppm of KNO3 and 4 mls of Flourish Iron. It is my understanding that the Terralit in my substrate has sufficient amounts of traces for the plants for at least a year (not 100% certain if this is the case however, so far, plants have been doing well).

On Tuesday I did a series of tests and tested the KH again, to my surprise, it was 110 ppm. Other than the nutrients for the plants, I didn't add anything to the tank since my first initial test. Then this afternoon (Thursday) I was curious to see what my KH was and tested again to find that my KH rose yet again to 130 ppm.

Does anybody have any suggestions as to why my KH is rising like this? Ever since I started adding the nutrients via PMDD, this has been happening. Prior to this I was fighting algae issues stopped nutrients altogether until I got a foothold on things as well as a better understanding about the topic (which I did thank goodness). Prior to this week, whenever I've tested the KH days after a water change, it has always remained the same.

Any ideas on what could be causing this?

Thanks in advance.

[wetmanNY]

Phosphates are counted as if they were carbonates in the alkalinity tests called "KH" tests. Consequently throwing off hopeful calculations of the dissolved carbon dioxide made by folks who want to calculate it by comparing "KH" and pH.

The alkalinity tests counted your enema as if it were part of the buffer-- which it now is.

Gives you an idea of whether or not your plants are actually using all that PO4.

[Lukara]

WetmanNY I appreciate your reply.

I would like to be sure I'm understanding what you said in your post...

Since Phosphates are counted as if they were carbonates for KH tests, the rise in my KH is due to the phosphates which I added. This would never have occurred to me, especially since only a few drops were added. Can I conclude that my plants didn't use the phosphate since my KH went up?

[wetmanNY]

Right, they are. Indeed it is. And yes.

In the acid/base reaction of the buffer, I believe that the alkalinity tests also register weak acids like humic acids.

I'm not a chemist. So if I'm being vague or misleading, it's always healthy to be corrected (probably by RTR) at moments like this...

[Lukara]

Thanks so very much WetmanNY.

See this is why I appreciate this site so much... The help here is excellent!