I got some of the Brightwell Magnesion a couple 3 months ago and gave it a try.
My older system before changing over to this last one was sucking down mag like crazy.
The salt I use is low in mag and I test all the time, even prior to water changes, but I like the salt (cheap).
I tried the Brightwell product and would get the mag up to 1300 only to have it back down below 1250 the next day.
I did this for a couple weeks and finally switched to Seachem Advantage mag. Lots cheaper and so forth.
Well, thought I would give the Brightwell a try again and it does work beautifully-once you get the mag level in your tank up and leveled for a week or two.
Now I add a half capful a couple times a week and the mag level stays just above 1300.
I believe that is what Chris told me also, to get the level up with a dry product then it is easy to maintain with the Magnesion.
Never asked about the science behind it and why this is so.
I do not have the brains for all that, just enough to sometimes be a danger.

works good here too.. I've just been too lazy to order some BRS 2 part (well 3 part).

I'd say Brightwells magnesion is the only product I wouldn't buy from them personally for one reason only. I'm weary about their ingredients label putting mag sulfate first and mag chloride second. In a saltwater aquarium adding sulfates to your tank can be bad , and a tank needs mag chloride...from what I've heard. I use blue life magnesium which doesn't have as high of a concentration (80000 ppm), but their main ingredient is mag chloride.

Anyone have any data to back the difference between sulfate and chloride in a sw aquarium?

Reef Central has some good info. specific to this topic. Many products such as Seachen Advantage Magnesium use Magnesium sulfate and Magnesium chloride (Magnesium sulfate is listed first and presumably makes up the majority). In theory, Magnesium sulfate could disrupt the tank water's ionic balance, but frequent water changes typically render this a non-issue.

I've used the Seachem product exclusively for over a year and have had no problems whatsoever.

this is something i've been trying to figure out -- seachem implies on their website that they are using magnesium chloride as well as magnesium sulfate -- but, on the bottle of the seachem reef advantage magnesium, it is only magnesium sulfate. what's the deal with this?

i just bought enough magnesium mix from bulk reef supply to make a gallon of magnesium, in correct ionic balance, for $9.99. much better price than anywhere else i've seen for dry ingredients, and incomparably cheaper than buying pre-mixed liquids.

something i read from randy holmes-farley on RC, the Brightwell Magnesion supplement incorrectly states how much magnesium is in the product by weight (it gives an impossibly high/apparently misleading figure -- like 43% or something -- which according to him is impossible).

i use Brightwell Magnesion in my tank and im pretty pleased with it.

my tank suffered from extremely low mg (under 1000ppm) and if i need to raise it up to 1300ppm, i would need a liter of kent tech M. so i opted for the powder stuff since i only need one bottle (300g) for my 100gal system. based on the dosing characteristics of Brightwell's Magnesion, a 300g jar would bring up mg level in my tank to exactly 1300ppm. so i figured ill slowly raise the mg lvl in my tank slowly so i split up the dose in 3 days. added 100g to the tank, let it run for a while and tested my mg. it was already at 1300ppm.

i monitored my tank's condition over the next few days and i already can tell my SPS seems to look better. mg level stayed pretty constant too.