Trying to understand the magnesium level of my tap water


Junior Poster
Dec 13, 2012
Sherwood Park, Canada
Hi folks

My local water quality reports give me a total hardness level and a calcium hardness level, but no magnesium hardness level.

Monthly average of Total Hardness 170 mg/L expressed as CaCO3
Monthly average of Calcium Hardness 117 mg/L expressed as CaCO3

I hit the internet thinking that if I had the total and the calcium there must be some way to calculate the magnesium.

I came across a manual for water treatment operators and got some information/calculations and ran the numbers. I'm curious if anyone here is experienced in this area and can tell me if this is a valid method for determining my magnesium levels or if I just wasted 20 minutes of my life :p

Total Hardness (CaCO3) = Calcium Hardness (CaCO3) + Magnesium Hardness (CaCO3)

Magnesium Hardness (CaCO3) = 170 - 117 = 53 mg/L (CaCO3)

Magnesium Hardness (CaCO3) / Equivalent Weight of CaCO3 = Magnesium (mg/L) / Equivalent Weight of Magnesium

Using the following Equivalent Weights

Calcium (Ca) 20.04
Magnesium (Mg) 12.15
Calcium Carbonate (CaCO3) 50.045

Magnesium (mg/L) = [Magnesium Hardness (CaCO3) * Equivalent Weight of Magnesium] / Equivalent Weight of CaCO3

Magnesium (mg/L) = [53 * 12.15] / 50.045 = 12.86 mg/L

I'm ultimately trying to figure out if I need to dose any additional magnesium and if my calculations are correct I think the answer is no.

Thanks in advance for any help!


Guru Class Expert
Oct 18, 2009
You are correct.
Won't hurt to add a little Mg.
Not really necessary


Junior Poster
Dec 13, 2012
Sherwood Park, Canada
Thanks for the reply.

I inherited some Seachem Equilibrium and was going to use that up on water change day to bring up the GH a bit. Glad to know know that the extra Mg won't hurt anything.



Junior Poster
Jan 9, 2013
New york
i think you are using too much magnesium, it may damage more if you increase on its quantity anymore
Last edited by a moderator:

Tom Barr

Staff member
Jan 23, 2005
SuomaSaariaho;94810 said:
i think you are using too much magnesium, it may damage more if you increase on its quantity anymore

I've had 4X this amount with out any issues with plants like R wallichii, L pantanal, Tonia, Erio setaceum.
High KH was another matter. Still, no reason to add GH booster, I'd just use K2SO4 maybe.....but you need a lot of fish waste and NO3 in the tap before that's required.

I'd like anyone making statements about high Mg causing issues to cough up some ppm's and species that are sensitive or affected.
One person did once, it was quickly falsified. My old Tap water had 52 ppm at Mg++. I had few issues related to GH(Ca or Mg), I have had issues related to high KH.

Not picking on you here or anything, but I've seen this myth go around for some time, till someone can show that it's true, and others can not falsify that claim, we should continue to refer to it as a myth, not a fact.
It's a rather easy process to falsify something, it's much harder to show it's true.
We just have to leave it alone till we can show it's true and not make the assumption.
Maybe it's true, but I've not seen any evidence in 20 years that gave any ppm's of Mg and the effects on Species A, B or C.
If 2-3 people have no issues at higher Mg, say 20-30 ppm, with several touchy species.......then..........well........


Prolific Poster
Nov 27, 2011
Sarasota Florida
Confused about Epson Salts and dosage

Speaking of Magnesium levels in tap water, I've been trying to figure out if there is a specific amount of Magnesium (Epson Salts) that everyone puts into their planted tanks to make sure their plants have enough.

I've got a 75 gallon heavily planted (in my eyes - considering my pruning) tank and I've found that it says on the internet to add 1/4 of a tsp of Magnesium for every 30 gallons and then double that in calcium. Is that correct? I'm adding about a teaspoon of Iron (Ferrous G) (test it using Seachem Iron test) and my plants eat it up every other day. I use Trace by Seachem at least three times a week. I do water changes weekly to keep nitrates down and read the Magnesium and Calcium should be added at the water change. Is that what everyone does to improve their magnesium levels in their water?

My water is super soft in the house (softener system - unfortunately - no way around it and city water is horrifying and toxic with Chloramines enough to kill a cow) and I know my GH and KH are low like 2 and 3 so I'm not sure how to manipulate it (magnesium). I guess I'm scared to go overboard so thought I'd ask if those numbers seem correct considering my GH/KH levels on our house water. As for buying water, so far my fish seem to be doing ok and my plants grow with no problems but I think I need to increase Magnesium and Calcium.