With the pre-mix vs. mixing your own, on a smaller tank like yours, I don't think it's worth the hassle, especially if you're happy with the pre-mixed water. You can easily add calcium, alkalinity and magnesium to the tank, if it's needed (test first). It gets a lot more expensive on a larger tank if you're adding a lot of supplements to a large volume of water, but with a smaller tank, it's more managable.