The Dutch translation of this article is being worked on, it is very inaccurate, incomplete and probably full of errors!
Translated in Dutch by Joost Abrahams

4.8. “Stability” isn’t Important

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Author : David Bogert

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Time To Read :
8 minutes
Difficulty : Level 6

Excerpt :

The going mantra is that tropical fish require “stability” and “constancy” of pH and temperature. Supposedly any rapid change will “shock”, “stress” and possibly kill the fish. This idea is simply a myth.
The going mantra is that tropical fish require “stability” and “constancy” of pH and temperature. Supposedly any rapid change over 0.2 pH or 2°F (0.5°C) temperature will “shock”, “stress” and possibly kill the fish. This idea is simply a myth, one of the many myths passed down by generations of aquarium owners. It is like saying humans will get a “cold” because they expose themselves to cold. In their native environment most fish are constantly subject to rapid and large changes in water chemistry and temperature. These changes do not “shock” them.

We tested this hypothesis by very rapidly changing temperature and pH for literally hundreds of fish. We had NO problems, NONE. No fish even seemed to notice the change. For this testing, which confirms the lack of sensitivity to rapid water parameter changes, go to this link:

4.8.1. Rapid Thermal and pH Changes in the Aquarium

Protomelas taeniolatius OB Red Empress
Protomelas taeniolatius OB – Red Empress

Yes, Fish Often Keel Over on Introduction to a New Aquarium

Now fish do quite often keel over and “die” when put in a new tank. We’ve all seen it. So it is quite natural that parameter difference became the scape goat. What actually happens is the fish equivalent of a heart attack. The fish is highly stressed, either by being shipped or by being chased around a tank for awhile. It is in a net and taken out of the water. It is then plunked into a tank. All of this can literally give a fish the fish equivalent of a heart attack.

The unfortunate part is that most of these dead fish aren’t dead! If the hobbyist just put some towels over the tank and left the fish in the dark, they will recover and be swimming merrily around in only a few hours. This is common in many fish. They “freeze” and become immobile after being handled too much.

OB Peacock
OB Peacock

Anecdotal Experience

We avoid anecdotal evidence as unreliable. But this is one instance where I will ignore that. I’ve been keeping up to twenty tanks for over 53 years now. I have NEVER paid very close attention to water temperature when doing a water change. I often do 90 to 95% water changes in tanks at 78 to 81°F (25 to 27°C) water with tap water that is 60 to 65°F (15 to 18°C) degrees. I’ve NEVER had fish deaths. That is a 13 to 21°F (7 to 15°C) swing in temperature.

One commenter to this website works for a local fish store. They have temperature mixing valves for doing water changes. As usual, these valves frequently malfunction and some very cold water ends up going in the tank. Try like 50°F (10°C) water!. They’ve never had a loss of fish.

Serrasalmus rhombeus Black Piranha
Serrasalmus rhombeus – Black Piranha

The Origins of the Myth

The author has been in the hobby for fifty three years. “Way back when” the going mantra was “old water is good water” and nobody did water changes. This resulted in aquariums becoming chemical cesspools. Very high nitrates, ammonia, nitrite, bacterial counts, bacterial toxins, etc. Fish have an ability to adapt to SLOW change in levels of chemical toxins. Their livers and their bodies just gradually ramp up their defenses.

When new fish from the store were put into this contaminated water the fish frequently all just keeled over and died. The only two parameters we measured “way back when” were pH and temperature. So these two parameters being different became the “obvious reason” for these deaths. The fish died due to toxins in the water, not rapid changes in pH or temperature. But we didn’t know that. And “stability” in water parameters slowly but surely became the new “mantra”. It was bad science and created one big myth.

picture of an aquarium fish Amatitlania sajica, T-bar cichlid
Amatitlania sajica – T-bar cichlid

What is Seen in Nature

Anyone who has ever taken a swim in a pond or a lake in the afternoon will tell you the water temperature at the surface can be considerably different from the water temperature just a few feet down.

Any biologist familiar with freshwater lakes will tell you the pH and temperature of the water changes constantly and amazingly quickly from one layer to another layer in the water. When scientists attempt to measure pH in a lake or pond with a probe they have some big problems. As the probe descends the pH can change up to two points in just a few feet. And the changes are unpredictable.

The pH in natural bodies of water changes rapidly as sunlight interacts with carbon dioxide and algae. Carbon dioxide in water is acidic. The carbon dioxide content of water varies constantly. The acidity goes up and down rapidly:

On a bright sunny day, in ponds with low alkalinity (less than 50 mg/l calcium carbonate) and intense phytoplankton blooms, pH may rise from a morning value of 7.0 to an afternoon peak of 9 or 10 due to photosynthetic removal of carbon dioxide from the water.

Masser, Texas A&M University, 2012:

When the water at the surface of a lake is 9 pH and 85°F (29°C) the water five feet down can be 7 pH and 75°F (24°C) degrees (Wurts and Durborow, University of Kentucky, 1992). And fish constantly go up and down through these thermoclines with no ill effects.

OB Peacock
OB Peacock

In the book “Limnology” by Wetzel, a study was done on a somewhat alkaline pond that was fifty meters in length. At 2:00 PM on a sunny day the pH varied very radically across the length of the pond, going from 7.6 to 10 in as little as two meters. And fish were swimming rapidly across these waters for the entire study with no apparent effect.

Water pH and KH
Water pH and KH

The alkalinity was going from 4 KH to 14 KH at the same time. This is very typical in nature and presents no problems with the fish in the ponds.

In the paper “Phytoplankton Photosynthesis, Micronutrient Interactions and Inorganic Carbon Availability in a Soft Water Vermont Lake”, Allen, 1972, the pH went from 5.7 at 10:00 am to 9.6 at 12:00 noon in an acid water lake.

TimepHDIC (mg/l)CO₂ (% of DIC)Photosynthesis (μg C//l/hr)
8:00 AM5,76,5815,2
10:00 AM5,72,67616
2:00 PM8,30,922,9
4:00 PM6,42,0540,4
Radical pH Changes in a Lake

This rapid change had no effect on the fish in the Lake.

picture of an aquarium fish Astronotus ocellatus Oscar Albino Ruby Red
Astronotus ocellatus Oscar Albino Ruby Red

In the Amazon, during the flooded season, these differences are much larger, going from 5 to 9 pH and 80 to 95°F (27 to 35°C) in a few feet (50 centimeter). Fish like corydoras constantly swim up and down and through these large pH and temperature differences. Fish like corydoras are NOT a “sensitive” fish.

In an alkaline lake such as Lake Malawi the pH changes are somewhat buffered and muted by the carbonate salts, but the temperature changes are not muted.

This is from an article:

Fish generally can tolerate a pH range from 6 to 9.5, although a rapid pH change of 2 units or more is harmful, especially to fry.

A rapid change of 2 units or more” is TEN TIMES the 0.2 units that many hobbyists ascribe to.

“Recirculating Aquaculture Tank Production Systems”, Masser et. al. Southern Regional Aquaculture Center, March 1992:

What people fail to recognize is how effective the skin and gills of the fish are in preventing “shock” from occurring. Acid water is rich in something called the “hydrogen ion” (H+1). Alkaline waters are rich in something called the “hydroxy ion” (OH-1). The gills and skin of a fish have something called a “bilipid membrane”.

Bilipid membranes are good at passing pure unchanged gases such as oxygen and carbon dioxide through the membrane. They are VERY good at PREVENTING charged ions from passing through the membrane. “Hydrogen ions” and “hydroxy ions” are charged ions and cannot pass through the membranes of the gills or skin. So pH changes cannot thus affect the fish.

Haplochromis Kyoga Flame Back
Haplochromis sp – Kyoga Flameback

Normally being cautious about pH and temperature changes does no harm and isn’t a problem (other than causing totally unneeded stress to newcomers to the hobby). But it becomes a problem when it is used as a reason for keeping fish in an aquarium filled with a poison. If a hobbyist finds a problem with very high ammonia or nitrite level, they should correct it immediately and rapidly.

A hobbyist shouldn’t leave their fish in poison filled water, they should do a 90 to 99% water change. We don’t drag humans “gradually” out of a carbon monoxide filled house. And there are no chemicals which “neutralize” ammonia or nitrite, that is profit motivated marketing hype.

Note this course of action is supported by the un-bagging procedure recommended by most experts, namely “cut, pour and plop”. This procedure takes fish from what is typically a very acid water to a more neutral or even alkaline water and what is sometimes a much different temperature very suddenly and it does the fish no harm.

For more information on how to unbag a fish go to this link:

4.9. Unbagging Fish

Dragonblood Peacock
Dragonblood Peacock

Another piece of anecdotal evidence comes from a breeder of tropical fish :

I’ve had my ponds go from 70 to 90°F (21 to 32°C) throughout the day and had zero issues or losses, and I’ve got 40 + tanks outside breeding different fish.

Sawyer Custom Aquatics:

Note that there is one caveat here. Temperatures below 65°F (18°C) can be deadly for tropical fish. I have had fatalities from that happening when doing a water change in Connecticut where the water temperature in the pipes in winter can be VERY cold.

The Lack of Importance of Stability in More Depth

For University Scientific Research which confirms the lack of sensitivity to rapid water parameter changes, go to this link:

4.8.1. Rapid Thermal and pH Changes in the Aquarium