said, no way you can have 8 dGH and 100 ppm TDS. One degree of hardness equals about 18 ppm (17.9 if you're fussy) of TDS, hence at a minimum:
5 dGH = 5 x 18 = 90 ppm
6 dGH = 6 x 18 = 108 ppm
7 dGH = 7 x 18 = 126 ppm
8 dGH = 8 x 18 = 144 ppm
So if you remineralised to a TDS of 100 ppm, you're probably at 5 dGH. Either your TDS meter or GH test kit has to be wrong, most likely your TDS meter.
The above TDS figures are also minimum values if all the SS GH+ minerals were just GH. But they're not - there is other non-GH TDS in there too! Not a lot (e.g. compared to Seachem Equilibrium), but definitely some. From my own measurements (I've been testing a lot lately after switching to SS GH+ for both my RCS and CRS tanks):
- remineralising to 6 dGH using 1.5g per 10 litres of RO gives a TDS of about 130 ppm
- remineralising to 7 dGH using 1.75g per 10 litres of RO gives a TDS of about 150 ppm.
both of which are more than the TDS you would expect if the SS minerals were only GH. I'm pretty sure my TDS meter is accurate as it's a new HM Digital model I calibrated at 342ppm NaCl last week, but obviously there is some variation between meters.
The instructions on the pot are pretty straight forward - add 3g per 20 litres (hence my 1.5 per 10 litres) to achieve a GH of 6 degrees. From my experience and measurements this past week, this is pretty accurate. So assuming you are using the SS GH+ minerals and RO / distilled water, ditch your TDS meter, get a set of accurate mini scales and:
- add 2.0g per 20 litres to hit 4 dGH
- add 2.5g per 20 litres to hit 5 dGH
- add 3.0g per 20 litres to hit 6 dGH
- add 3.5g per 20 litres to hit 7 dGH
Or once you have a good, calibrated TDS meter, aim for 130 ppm and you should be at 6 dGH (which is what SS seem to be recommending for CRS).
Hope this helps! Regards, James =)