Hello, so got a bit of a strange issue and was hoping someone would have some input. I have a long time planted tank, 3+ years and have been running co2 for the last few months after rescaping. The tank is 40 gallons with an additional 10 gallon sump and the substrate for the past 3+ years has been fluval stratum. Because of this the ph in the tank has always been in the 6.5 range with no kh even though the gh is very high (and so is the kh from the tap). A few days ago while doing water tests I noticed that my ph was lower than normal and that nitrites were starting to show up on a test strip which has never happened before after doing a liquid test they were in between 2 and 5 ppm and the ph was around 6 with no ammonia. So I did a large water change and added prime, stability and purigen assuming that somehow the bacterial colony took a hit after cleaning the sump sponges. It’s been about 3 days of 25% water changes after the initial 50% one and the nitrites are rebounding each time. Since the gh in my water was already so high I was reluctant to add crushed coral to bring the ph up but I have roughly 1lbs in the sump now. I am assuming that the ph dipped too low and the nitrifying bacteria colony died and crashed the tank. I guess the question here is if that makes sense or if there could be some other cause.
Tank stocking
15 cherry barbs
2 bristlenose plecos 1 adult 1 juvenile
5 or so trapdoor snails
1 dwarf gourami
Thanks!