My experience with ProMix would lead me to believe that it will take about 1/3 of the pot size in water to saturate the media. A 15 gallon container will need about 5 gallons of water before you'll have run off. That gallon you're feeding every 5 days or so is coming nowhere near wetting all of it. If only a portion of the media is wet, the roots aren't going to grow into the dry zones. You end up with a giant pot that is no more effective than a much smaller container. Your root ball is probably more centralized in the area around the plant, more where you water, maybe 5 gallon container sized. You'll hear people say to flush with 3x the volume of the pot. For you that's 45 gallons. That's a lot of water and maybe a little excessive for your situation. I just ended up semi flushing my Purple Kush this AM. She was starting to show some clawing, which is an issue I've never had before. In my 5 gallon pot I ran 4 gallons through her. After about a gallon and a half I had run off (since she was still damp from a watering a couple days ago). The next 2 1/2 was to rinse things out. I ran a weak feed solution (2/3 strength) through it the whole 4 gallons. If you did that you'd probably be burning a lot of nutes. So where does that leave us? Assuming your roots are only using about 5 gallons of your pot right now, and haven't filled that big pot, I might try running about 10 gallons of tap water through it, then follow with 4 gallons of your new feed mix. That pot is gonna be heavy after that! Good news is that she probably won't need a drink for 10 days after that.
