Aquarium GH and KH

Question
QUESTION: I am having some problems at the moment with GH and KH. Mainly about how to evaluate the results from the test kit.

According to the test kit it took 9 drops of the GH regeant to change colour from pink to blue and it took 4 drops of the KH regeant to change the colour from blue to yellow.

Does each drop of regeant equal 1 Degree of Hardness (dh)? I have heard that you need to times the number of dh by 17.9 to found out the GH and KH in mg/l, but I am unsure if 1 dh = 1 drop of the regeant.

For example:

My GH = 9 drops. Does that mean its 9dh = 9 x 17.9 = 161.1 mg/l?

My KH = 4 drops. Does that mean its 4dh = 4 x 17.9 = 71.6mg/l

Also is mg/l the same as ppm?

Thanks for your help:)

ANSWER: Hi George,


Yes, you are correct. With amost ALL  KH/GH tests I've encountered, the number of drops it takes to get the water to the desired level (yellow), is your KH/GH reading. For example, if it takes 4 drops to get to yellow, then your KH reading is 4.

You are correct, you multiply the rating in dh units by 17.9 to get the mg/l reading. In aquarium terms, mg/l and ppm measure the same thing, and are 'equivalent'. It is only equivalent in measuring water, however.

Good Luck, and happy Fishkeeping!


---------- FOLLOW-UP ----------

QUESTION: Thanks for that:)

So to recap:

Drops of regeant = dh reading = x 17.9 = result in mg/l (ppm)

One other measurement that has been bothering me is TDS. I have found resources on the net very confusing about this. How does TDS relate to GH/KH? If I have a GH of 10dh (179ppm) how much TDS does that equal?

Thanks again

Answer
Hi George,

Yes, that's right.

Every drop of reagent = 1 dh
dh x 17.9 = reading in mg/l.

TDS is the total amount of charged ions, minerals, salts or metals dissolved in a given volume of water.  It's expressed in units of mg per unit volume of water (mg/L), also referred to as parts per million (ppm).

TDS readings are not necessary in aquariums, and very few people measure the TDS in the aquarium. There are no commonly available tests to use to measure it, and TDS is rarely used in marine (saltwater) aquariums, and never used in freshwater aquariums.

Don't worry about TDS, as long as you GH and KH ratings are fine.

Good Luck, and Happy Fishkeeping!