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I'm not sure where those ratios came from but I think the rationale is that the Ca:Mg ratio in freshwater is usually about 3 or 4, depending on who you talk to. In fact, the natural ratio is pretty variable.

If I were to pick one composition to use as "average fresh water" then it would be the world average river water. That is the average composition of freshwater entering the oceans. The Ca:Mg ratio in average river water is 3.66. By contrast, up until a few years ago my tap water had a ratio of 14 and now it's running about 5.

A few months back I pulled together some water analyses for a talk. In those analyses some of the Ca:Mg ratios were:

3.25 headwater mountain stream
2.68 Lake Malawi
2.09 Lake Victoria
1.59 black water stream in Georgia
0.32 Lake Tanganyika
0.31 sea water

I have no reason to believe that plants are particularly sensitive to the ratio. The best guidance I've read in hydroponics literature is that the magnesium concentration should not exceed the calcium concentration. That is the guidance I pass around. The guidance is based on the belief that when the ratio gets too far out of proportion then magnesium can block the uptake of calcium. Similar guidance applies to potassium:calcium ratios.

I've been supplimenting magnesium in my tanks for years now. If I don't suppliment it then I have deficiency problems. I don't think the Ca:Mg ratio has anything to do with the problem. It's more likely because without supplimentation the tanks get less than 1 ppm of magnesium.

Roger Miller
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