Ca to Mg Relationship and dosing to get there


Junior Poster
Sep 8, 2005
Arlington, Texas
I am relatively new at this chemistry thing (not my favorite subject in college but I did pass) and I have a vested interest in this Ca/Mg relationship. According to my calculations, I need to be adding about 2.5 teaspoons of Epson salts to my 30 gallon tank each water change to get a 4:1 relationship of Ca to Mg (apparently the desired ratio).

Hopefully, my calculations are screwed up as that seems to be alot. Anyway, here's how I came to that conclusion. According to my city water company, I have 42 ppm of Ca in my tap water from a recent test and my GH tests at 100 ppm with 2 different testing kits. I downloaded a MgGH calculator program by Stuart Keeler (assumption here that this calculator is accurate) and after plugging in my Ca and GH values, it calculates I have 0 (actually below 0) Mg in my tap water (which I believe according to the symptoms my plants are showing - old leaves have yellow/brown spots which develop to brown outlined pinholes). According to the fertilator, I need to add 2.5 tsp of Epson salts to get about 10.5 ppm of Mg to meet the 4:1 ratio. However, I must say that those old Ludwigia Leaves are the largest I have ever seen. My bacopa leaves are quite large also and have no problems except not turning copper in the top 2 inches. Some GSA algae present on glass and on the old leaves. Altenethera Reneicki showing show new growth but stress overall on the whole plant.

I guess I have 2 questions. Is that correct, that I need to add that much? And if so, should I do it all at once or break it up over multiple days?

I am dosing EI with Macros and Micros of Excel, Flourish and Flourish Fe, weekly 50-60% water changes, pressurized CO2 at 40ppm (KH of 4, Ph of 6.5, temp at 82 and all plants perling and growing (using CO2 misting system with my Duetto internal filter for about the last month) but only the top 2-4 inches are healthy, except for my Bacopa C. which is superstrong an growing 5 inches a week with large leaves), 4.5WPG for 6 Hours, 2.5WPG for 4 hours (2 lights, 2 timers), medium fish load, medium to heavily planted with Bacopa C., Rotala Rotundafolia, Hygrophila Difformis, E. Tenellus, Ludwigia Repens, Java Fern, Alternanthera Reneicki. Nitrates are over 20ppm and don't go down much in a week. Phosphates are at 2.5ppm and get used at a rate of about .3 to .4 per day.

Thanks ahead of time for any thoughts/ideas, etc., I need some info to learn more about this relationship with nutrient uptakes, etc.

Mike Herod

Tom Barr

Staff member
Jan 23, 2005
Re: Ca to Mg Relationship and dosing to get there

I've mentioned that a ratio in and of itself is not particularly important. If you have a limitation, then that is important.

For a 30 gallon tank, adding 1/4 to 1/2 teaspoon once a week of MgSO4 is more than enough to reliveve the issue.

But with GSA and other issues........CO2 and PO4 are the other issues, if you do EI, you can rule out PO4.

So you can rule out Mg quickly and then are simply left with CO2.........

Tom Barr