Brita filters, most pitcher type filters, use different ion exchange resins. It's very common for them to add ppms to your water-these resins also alter the PH depending on the type used, as you noticed. Britas are notorious for dumping aluminum back into your water if you use it for too long. It's better not to use them with plants. If your goal is to reduce chlorine/chloramine, you can use regular old ascorbic acid (vitamin C). I buy 1 lb bags of bulk ascorbic acid on amazon for about $12 and you just need a tiny bit per gallon, something like 30 mg. For a 5 gallon bucket, I use about 1/4 to 1/8 teaspoon, which is more than needed but doesn't hurt anything. If you want to dechlorinate a bathtub full of water, use 1 Teaspoon. Also, the best "organic" PH down is Citric Acid, which you can also buy in bulk on Amazon and it lasts forever. I've been using the same 2 lb bag for at least the last two years and I'm maybe halfway through it. How much to use depends on the alkalinity of your water, so you need to experiment, but I would start with a small amount-I'm guessing you'll probably need somewhere around 1/8 teaspoon or less for your water, as a starting point. Just play around with it and you'll figure out the right dosage. FYI the cheap PH and TDS pens can be wildly inaccurate, and if you get them wet, they're done for. You'll probably need to spend around $40 each for accurate mid range pens, and something like $80 each for Bluelab pens. PH test kits are a good choice to verify the accuracy of your pen and don't cost much. You can also check your TDS pen against a reference solution to see if it's accurate, or buy a bottle of distilled water and check it against that.
