Low KH and pH in a BN Pleco Growout Question

[Stef]

Hi everyone. So I have a 20 gal long that I’m using as a growout for an accidental brood of BN Plecos born in April this year. There’s about 80 plecos remaining (I’ve been donating to lfs and anyone who wants one). I know it’s overstocked. I do water changes every week. I have 3 hob and two sponge filters. There’s next to no plants other than a stray anubias, floating rotala clippings, and an entire pothos split into mesh bags. 
 

Today when doing a water change, I used a test strip to test water before draining half the water. This was the first time I’ve noticed how low the pH and KH is. The KH was at 0 and pH at 6.4. My tap water is at pH 7.2 - 7-4. 
 

So my question is what causes the drop in pH and KH if other parameters are ideal range?

 

The first pic is before water change. The second pic is the tank. The third pic is after the water change. 
 

After the water change, I also used the liquid test for KH and got it to 4 drops to change color. 

[nabokovfan87]

On 12/19/2022 at 7:03 PM, Stef said:

So my question is what causes the drop in pH and KH if other parameters are ideal range?

Waste. KH is a series of specific elements in your water and having that present gives you a "bigger trashcan" before you see something like a PH crash. Once you have a spike in waste, chemical changes happen and ions are bonded/reacting. As a result your KH drops, and as a result of that your PH drops. 

https://www.aquariumcoop.com/blogs/aquarium/ph-gh-kh

On 12/19/2022 at 7:03 PM, Stef said:

Today when doing a water change, I used a test strip to test water before draining half the water. This was the first time I’ve noticed how low the pH and KH is. The KH was at 0 and pH at 6.4. My tap water is at pH 7.2 - 7-4. 

You'll also learn a bit more about your water by performing an off-gas test.  Some water can alter it's parameters after it comes out of the tap. You'll usually see small changes in results. It also helps give you an idea of what your water changes are doing in your tank and how something like old tank syndrome can become prevalent in tanks with water quality issues.

1. Take a sample of water from the tap, test it for everything you can.

2. Aerate that same sample for 24 hours with an air stone, then retest. (This is what your water parameters are during water changes)

3. Compare test #2 results to your tank and this gives you an idea of what water changes are doing to your water chemistry.

[rockfisher]

You could be just fine. My water always has 0 Kh. I breed several breeds of fish. I do little maintenance with no problems. You are doing lots of water changes I think you will be ok. I am of the school that the numbers you have are what you have. I monitor them and when there is a change in the norm then I get concerned.  Chasing numbers is hard and often does more harm than good. If your fish are not dying and you aren’t seeing problems I would not worry about. My water test look almost the same except I have almost 0 nitrates. Plants deal with that and think layer of substance. The bacteria that breaks down nitrates is anaerobic(without oxygen). My ph also tends to be 6 or 6.2. The fish have made it since April I would not sweat it to much and you are getting rid of them so that will improve things. You may want to sell some on line on eBay, aquabid, or here.  

[Stef]

Thanks @nabokovfan87, so even though Ammonia and Nitrite are zero, the presence of waste, and in my case lots of baby BN poop, and only for a week, still causes a crash?

 

“Think of KH like a trash can. The higher the KH, the larger the trash can. If we overflow that trash can, then a pH crash occurs. Therefore, people with low KH in their tap water often use crushed coral to gradually raise the KH (or increase the size of their trash can) and prevent pH crashes.”

 

Based on above excerpt from the article, the way to stabilize is crushed coral. Since my situation is temporary (i.e. Pleco numbers will steadily decrease as they are given away), is coral or more frequent water changes the best course of action?

[nabokovfan87]

On 12/19/2022 at 9:14 PM, Stef said:

Thanks @nabokovfan87, so even though Ammonia and Nitrite are zero, the presence of waste, and in my case lots of baby BN poop, and only for a week, still causes a crash?

Yes. Correct. I have very similar, almost identical water to yours. I dose in some alkalinity buffer in my tanks to get the KH from 40 up to 60-80 ppm to give it a little more stability. Another, more safe way to do that is to use crushed coral, 1lb per 10G.