Why does the pH level in my koi pond fluctuate?

Koi Fish Pond pH Level Guide

How can I prevent a “DEADLY” pH crash in my koi fish pond?

The pH level of your koi fish pond water fluctuates depending on the time of day you test your koi fish pond water. Testing your koi pond water in the morning for pH levels will give you a lower pH reading than testing pond water in the afternoon. The KH or “carbon hardness” of your koi pond water is one factor that will determine just how much your pH level will fluctuate within 24 hours.

Every koi and fish pond is unique and no two koi ponds are exactly the same. pH readings for your koi and fish pond could range from 7.6 to 8.1 or 7.1 to 7.5 or even a bigger range between the high and low pH may occur. This is the unique daily pH cycle of your koi pond. One word of caution: don’t fall into the false belief that your pond water must stay below a certain pH reading, here’s why.

You’re koi fish pond water is going to have a natural pH high in the late afternoon, this is normal. The very worst thing you could do is panic, because according to what you read your pH must be kept, lets just say below 7.5 and yours is 8.2. Now you want to add a chemical to bring your pH down, which is an acid.

WRONG! By adding the acid you just shocked your koi and pond fish by interfering with the natural pH cycle of your koi pond! Sudden pH changes (especially lowering the pH) will stress your koi and pond fish which can lower their resistance to bacterial infections. Plus, adding the acid to lower your pH in the afternoon could cause an overnight “pH crash” which will kill your koi and pond fish. Yes, you could have a pond full of dead koi and pond fish in the morning!

Here’s the reason why: Because it is natural for your koi pond water to have a much lower pH at night and the addition of the acid will cause your pond water nighttime pH to drop to dangerous levels. Your koi and pond fish have adapted to the natural slow pH cycle (up and down) of your pond water and it does not stress them.

What causes the natural pH swings in my koi pond? No tech talk!

During the day plants and free floating algae in your koi pond produce oxygen and take up carbon dioxide CO2. The decrease in CO2 increases the pH in your koi and fish pond, caused by photosynthesis (sunlight on plants). At night the plants and algae take up oxygen and produce CO2 which decreases the pH in your pond water.

That’s why your tests show a low ph and low oxygen level in your pond water early in the morning. Plus, all the bacteria, your koi and pond fish and the sludge on the bottom of your koi pond all have an effect on the oxygen and CO2 levels of your koi pond. Oxygen levels in your koi pond?.

How can I prevent a deadly pH crash in my koi pond?

There are the two life supporting elements in your Koi pond that can be related to your home.

A home without a solid foundation will fall. The pH level in your Koi pond will fall without a solid foundation of carbonates (KH) carbonate hardness. Good koi care means keeping the (KH) at a safe level in your koi pond. A KH reading above 140 or better is the best for your koi pond water.

A fall in the pH of your koi pond stresses your koi goldfish and if low enough can even kill them. pH is measured on a scale of 1 to 14. A reading of 7 is neutral, below 7 your water is “Acid” above 7 your water is “Alkaline”. A good reading for a koi pond is 7.5 or better. Koi pond fish and the good bacteria in your filter thrive in alkaline pond water.

The carbonates (KH) in your koi pond determine how stable your pH is. Your koi and pond fish and the bad and good bacteria need and use up the carbonates in your koi pond everyday. Without a good carbonate base in your pond water, your pH will fall below 7 and your pondwater will be in the “Acid” range. I don’t believe in lowering the pH in an aged koi or fish pond with a ZERO ammonia reading unless the pH is very high (9 or above).

You can raise your pH in your koi and fish pond very quickly if you see a “pH crash” coming, without hurting your koi or pond fish, however, if you try to lower your pH and it drops even two points within a few hours, your koi and pond fish can become VERY stressed and could even die.

Note: A high pH in a koi fish pond with a high ammonia reading can be a very serious problem, because ammonia is more toxic with a high pH. I still would not drop the pH. I would do massive water changes to bring down the high ammonia reading. And/or add a good ammonia remover like our Dechlor & More ASAP.

For good Koi care: You want a high enough reading of KH in your koi fish pond to maintain your pH level of above 7 overnight. If your pH is dropping below 7 you must add a buffer like our Buff-it-Up which contains life supporting carbonates to give your pH a good foundation of KH. A good solid foundation of carbonates will keep your koi fish and your life supporting “good bacteria” healthy.


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