When I first started I was getting RO water from my LFS. At £6.50 a throw, it soon adds up. Then I invested in an RO machine and saved myself a fortune. It paid for itself within a couple of months. I got a bargain on an unused 6 stage machine. If memory serves, I think I paid £67 + shipping.
Definitely worth the investment. I also drink the filtered water too. The "waste" water, which is still much cleaner than tap water, goes into a big rainwater butt outside, where I have 4 fancy goldfish. I rescued them from Petsmart lol. They get nice clean water and eat all the mosquito larvae and get flake food to boot.
Yeah I use Seachem too. The Parcel Force driver was complaining about the weight of the box he delivered to me. Well it did have 8kg of sand in it and a 20kg bucket of Seachem Reefsalt lol.