Wacky pH/KH crash after vacation

[MrGrieves]

I have a heavily stocked, moderately well planted 20 gallon tank. It is filled with guppies, 1 hillstream loach, 1 dwarf gourami and some amanos. This tank is 5 years old and quite stable. Typically parameters look like:

pH 7.4
GH 8
KH 4
Ammonia 0
Nitrite 0
Nitrate 40-70ppm

I just came back from vacation and wanted to see how the nitrates were looking as that is the parameter I am usually chasing with water changes. To my surprise my pH and KH have both crashed. Current parameters are:

pH 6.5
GH 11
KH 0
Ammonia 0
Nitrite 0
Nitrate 65ppm

I have no idea what has happened. Fish from what I can tell all look fine. Some plants are looking not great but not sure if related: crypt wendtii with a bunch of pale yellow leaves, java fern and anubius have multiple leaves with holes with black edges around them. 

I had our pet sitter add a pinch of my usual flake food every day while I was gone. Quite possible she overfed as she completely fouled up my other tank. The only other thing I did differently was add a Zoo Med Nano block for my amano shrimp. I would assume the nano block would have had the opposite effect on my KH/pH though.

In case it is relevant I also have a small bag of API Nitra-Zorb in the filter to help control the nitrates but I have been running this on and off for months. I did recharge the bag right before I left. 

Any ideas why this could have happened? Also how do I deal with this? Water change seems obvious but do I have to do so cautiously to avoid a pH shock to the fish?

Any advice appreciated!!

[Fish Folk]

This is pretty common, actually. KH at 4 is your temporary hardness -- your buffer. If you're not a chemist . . . maybe think of it like a few spoons of Baking Soda dissolved in your tank. When food breaks down and plants decay, there is a creep up in humic acid. Think of this like . . . dilute cider vinegar in your tank. Well, the acid and base subtly react like those old "volcano" projects in grade school. The KH gets neutralized (this is why you're reading KH = 0), and the result is that your pH begins to drift dramatically low. This happens consistently in _all_ of my tanks that don't have ample supplies of limestone, or crushed coral liberally added to the substrate.

Good news is that you're probably set up with ideal conditions now for South American species -- dwarf cichlids, tetras, Discus, etc. But African Rift Lake species would probably not thrive. 

[MrGrieves]

On 8/6/2023 at 8:34 PM, Fish Folk said:

This is pretty common, actually. KH at 4 is your temporary hardness -- your buffer. If you're not a chemist . . . maybe think of it like a few spoons of Baking Soda dissolved in your tank. When food breaks down and plants decay, there is a creep up in humic acid. Think of this like . . . dilute cider vinegar in your tank. Well, the acid and base subtly react like those old "volcano" projects in grade school. The KH gets neutralized (this is why you're reading KH = 0), and the result is that your pH begins to drift dramatically low. This happens consistently in _all_ of my tanks that don't have ample supplies of limestone, or crushed coral liberally added to the substrate.

Good news is that you're probably set up with ideal conditions now for South American species -- dwarf cichlids, tetras, Discus, etc. But African Rift Lake species would probably not thrive. 

Thanks for the reply! I do have a chemistry background so I follow what you are saying. However just to clarify - this happened over the course of a week (KH from 4->0), is this typically what you observe? I do not normally have a source of CaCO3 in the tank but I do ~weekly water changes so my KH usually matches my tap water. I'm unclear on why this would happen so suddenly. 

[nabokovfan87]

On 8/6/2023 at 6:07 PM, MrGrieves said:

Thanks for the reply! I do have a chemistry background so I follow what you are saying. However just to clarify - this happened over the course of a week (KH from 4->0), is this typically what you observe? I do not normally have a source of CaCO3 in the tank but I do ~weekly water changes so my KH usually matches my tap water. I'm unclear on why this would happen so suddenly. 

In my case (we might actually be from the same area) there is a bit of a "if you don't keep on top of it you're going to crash" when it comes to KH.  Mine is 3-4 on any given day.

Higher stock on the tanks, waiting to do water changes, all of those organics bind with the KH molecules and that leads to KH and PH crashing.

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

In my case, I have some seachem alkalinity buffer on hand (there's some common kitchen items that people recommend, but I just don't know what they are off-hand) and  those can be used in the same way.  Try to get your KH to stay above 4, keep on top of water changes, no big deal. 

It's just the type of thing where I highly recommend having a GH and KH liquid test kit on hand for monthly checks just for your own sanity and keeping track of how things change over time. To be fair, I believe you already have one, but just want to mention it.

More organics in the water, waste and other things, uses up the KH, that causes PH to drop.  My tap is ~6.8 and can easily drop to 6.0 if I'm not careful.  I keep the tanks light, still something I battle with, but it's extremely manageable.