The TWP is very economical if your local water is good. However, 300g is not something I would believe, since the unit is rated to a maximum of 150g.
It is still the most economical unit for me, as I can get 80plus gallons per unit = $20$US approximately for a refill ?? Used to get much more, but recent water scares in the country have obviously resulted in more additives......
The important thing to have, and I am sure you have already since you have a reef, is an
alk kit so you can test the output of your TWP after a certain amount, as per the instructions. The color can be hard to read.