If you say I need 10 grams to get 10ppm, does it mean I have to put more than 2 teaspoon?
Hello mate, like
@ceg4048 says there's no need to worry too much about magnesium as its one of those elements that you need some presence of and the plants will use it so you get a bit of tolerance and no need for being too exact. Ceg doesn't worry about the amount of salt because he has ran tanks with extreme high levels just as an experiment and found no detriment so best practice is to have more rather than less to be on the safe side. Too much of everything is better for plants than not enough of one thing, if something is low the plants will suffer. However, I appreciate that people will worry and when they are trying to get things right they want to understand why they are doing something rather than indiscriminately chucking stuff in their tank even though Magnesium is something that you can indiscriminately chuck in your tank within reason
If you haven't already done so have a read of the
<UKAPS Article> based on the findings of
@plantbrain (Tom Barr) and wrote and explained by ceg (Clive see above) essentially the idiots guide for the likes of me it comes down to making sure plants don't get deprived of any nutrients during the course of a week between water changes. The parameters to make this happen are recommended as follows.....
Nitrate (NO3) 20ppm per week
Potassium (K) 30ppm per week
Phosphate (PO4) 3ppm per week
Magnesium (Mg) 10ppm per week
Iron (Fe) 0.5ppm per week
Dosing traces and NPK on alternate days so the Iron doesn't react with phosphate.
If you want to know how to reach these levels I tend to use
<James Planted Tank Calc> for easiness. Just put in your tank volume and for dry salts use 1 in all the other boxes and teaspoons or you can use grams if you have access to some digital scales but as Clive says it's not exact science.
I have roughly 100ltr tank so I tend to use 2 teaspoons, I also have digital scales and using a 5ml medicine spoon I found that each level spoon of epsom salt was about 5gram on average so I put the scales away and went with that. If I find my GH is rising significantly over time I either change more water that week or one week do a spoon less. It isn't that important.