Ooooooohhh, now I know what you are talking about - two types of Mg supplement - one being Kent, the other Seachem
Geez, some people are just naturally slow
Don't know anything about Seachem's product, only Kent's.
Yes, Ca and Mg on that calculator are measured only in ppm, as you state. Alk has the three measurements from which to choose.
Danny, a couple of things to consider here:
1. Did you use the tank volume calculator using inside measurements for an accurate volume of your tank?
2. Don't dose the Mg supplement all in one day. Spread it out over 2-3 days. For instance, I use a 10 ml plastic syringe for dosing so I can record how much I dose at a given time. In my 37g (which has a volume of only 35.5g according the volume calculator), I add 30 ml at a time. I might dose every 1-4 hours throughout the day, depending on what my day is like.
3. Measure Mg at some point to while dosing to see if you are actually overdosing or on target.
4. Record all test results and how much of what you dose so you can go back and observe patterns of use of alk, Ca and Mg. I also record pH before and after dosing (for buffers), time of day, and occasionally nitrate.
Dang, that's more than a couple of things, now isn't it
