I tried mixing my own. It's not difficult, but it is a pain in the rear. I'm not one to waste resources, but I still have a quarter of a bale of peat moss and half a bale of perlite, a bag of garden lime and shitloads worm castings sitting around. So much easier going with a quality mix to begin with. But likely I'll use some of those ingredients to adjust the Miracle Grow 6 month I'm in right now. (which, I don't recommend for beginners)

I use an aquarium ph test kit, which does the same thing as the pool kit. Both are un-usable for nutrient-tinted or runoff water. But using a commercial potting mix, the buffering they add usually lasts about 6 to 8 weeks. Normaly I use a ph pen, but recently I dropped it and cracked the housing. Not waterproof any more.

I'd start looking for another potting soil soon, and transplant asap. I'd avoid Supersoil though. I recently was forced to try it, and had inconsistent buffering, and there's no perlite in it to 'lighten' the soil structure which allows proper drainage. Plus, it has chunks of wood and bark that once you get 'em saturated with water, retain that moisture waaaay too long. (not a good thing for cannabis roots) Try and find one that's not so heavy-handed with the nutrients, if possible. Best case is to fine one that isnt pre-ferted, but has ph buffers and perlite. That way, you are the one in control of the nutrients.

But if you go this route, (non time-release soil) you'll need a solid nutrient system. I use a 3-part nutrient system, and their website has a weekly application chart. Almost too easy to fail. :thumbsup:
(Fox Farms, btw)