Established planted tank crash

[Tuppins]

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! 

[Galabar]

You mentioned "no KH" in the tank (but high KH from the tap).  How often do you do water changes (and how much)?  With high KH in the tap water, that should probably be enough for your tank.  However, if you never (or very rarely) introduce new tap, you can end up with "old tank syndrome," with nitrifying bacterial needing KH (Carbonate) to process ammonia, but not having any. 

Also note that CO2 will lower your pH.  However, that is different (I think) and wouldn't cause the same issues as "actually" having low pH because of a lack of buffering.

https://www.epa.gov/sites/default/files/2015-09/documents/nitrification_1.pdf

 

Edited December 7, 2023 by Galabar

[Tuppins]

I usually do water changes once nitrates reach around 50 which works out to around once every 5 days or so. I also know that the substrate strips out kh somehow but I figured after such a long time the buffering would be depleted. I’ll look into old tank syndrome and see if that fits though thanks for the suggestion!

[Galabar]

On 12/7/2023 at 7:31 AM, Tuppins said:

I usually do water changes once nitrates reach around 50 which works out to around once every 5 days or so. I also know that the substrate strips out kh somehow but I figured after such a long time the buffering would be depleted. I’ll look into old tank syndrome and see if that fits though thanks for the suggestion!

Yeah, that substrate will strip out the KH for a while.  I'm not sure how long it lasts.

[Mmiller2001]

Can you explain what’s “crashing”? If you are referring to a .5pH drop, that’s not a crash or anything to worry about. I’d be more worried about going from 5ppm NO3 to 50ppm in 5 days. Something is wrong if that is happening and I would address this problem. 

Edited December 8, 2023 by Mmiller2001