I'm just going to start with a story before I get to your issue since I believe it'll help a bit.
I bought a tropical semi-agressive tank on boxing day, taught my mom what to do so when I went back to University she wouldn't kill the fish. Came back on reading break and OH MY GOSH! pH was 5.6, I cleaned the tank and red debris came flying up! She had been over feeding and not changing the filter cartage. So I went out, bought an emergency API kit (I only brought a Nitrate one I use on my betta here), and new filter cartridges. When I went to go do my free water test at petsmart, they said it the nitrates were 80ppm.... and everything else was fine. Did it myself, Ammonia was HIGH, everything else was in a decent range. So, I did a water change (75% the first day, 50% every 2nd day until I had to leave). I managed to get the pH up to 6.8 which my angels like.
I also used a triple dose of Prime! Helped a lot after I read that tip on the back of the bottle.