· snails are your friend
You have aqua soil so keeping 0 dKH is going to be best. I aim for 5-7 GH, using a GH only mineralizer. 279 seems awfully high, in fact I'm not sure it's possible at 4-5 GH and 0 KH. TDS pens are finicky and need calibrating from time to time, my guess is that's where the reading is faulty.