Hi, all! I'm a new user on the forum, and I joined because I am not able to figure out what's going on with my tank. I was hoping someone could help me with this. (Bear with me; there's quite a story to this.)
A little bit of background: I have been keeping fish for 12 years now. I have a 20H freshwater aquarium that was replaced because my old one sprang a leak. It used to be a community tank with cories, snails, and a betta. However, right as the leak happened, all of my cories and snails died within days of one another. The leak was very slow, so they hadn't been transferred to the new tank yet. The only surviving fish, miraculously, was my betta. I replaced the tank and put some new substrate (old substrate was ~3 years old by that point), but kept all the old filter media and decorations to preserve bacteria. The only new additive was the substrate. The new tank has a sponge filter, an oversized HOB filter, and sponges everywhere to facilitate bacterial growth.
I believe the old tank developed old tank syndrome from old and compacted substrate, because the pH was scary low (it didn't even register on the test kit) and ammonia was around 4.0 ppm. My betta, somehow, still hung in there. Unfortunately (but expectedly), the low pH and high ammonia transferred to the new tank. After months of a painstakingly slow cycle and lots of water changes that made our rose bushes very happy, the ammonia level finally dropped to zero (no higher than 0.25 ppm) and stayed that way. I was able to scale water changes back to every other week (what I found works best with that tank) and parameters always read normal. However, the pH was still very low, despite normal parameters everywhere else (ammonia, nitrate, nitrite, etc.). As a result, my aquarium plants basically all died off and any left were melting and/or covered in algae. This, of course, caused the ammonia to rise a little bit.
To help with this, I bought crushed coral to raise the pH, as I heard it was a safe and natural remedy for low pH. (I installed a small HOB filter with an extra sponge for bacteria to use for the coral.) Right as I started this, my ammonia was at 0 ppm. I watched the water cloud up, so I assumed it was doing its work. Plants were also immediately doing better. I did a test the other day before my regular water change to see where things were at, and the pH was at a nice 7.4 ppm. The ammonia, however, was completely maxed out (8.0 ppm+). What I thought was clouding from the coral was actually a dangerous ammonia spike. I couldn't detect it because I also have Indian almond leaves in the tank, which turn the water orange due to the tannins (same color as ammonia clouding). My betta was also perfectly happy and energetic and the clouding actually went away on its own before the water change, so I never knew anything was amiss.
I immediately did a large water change, but the ammonia has been steadily climbing ever since. I will continue to do water changes to drive the ammonia level down. Does anyone know what's going on here? Did I just undo my previous cycle on this tank? From what I understand, it would seem to be ammonium-to-ammonia conversion from the higher pH, but I want to be sure. I also ask because, in all my years of keeping fish, I have never dealt with this before. For the record, my betta is somehow still hanging in there, doing his little happy-dances, exploring his tank, and swimming around like a happy camper...
Sorry for the novel, but any advice here would be greatly appreciated. Thank you all!