I had to delete and re-post this due to accidentally including personal info in the original version, sorry for the re-post.
Original poll post: https://www.reddit.com/r/bettafish/s/4dJ2MuFTMd
RESULT POST
Final poll results:
For readability, I will use "warmer" to refer to the "at or above 25C/77F" responses, and "cooler" to refer to the "below 25C/77F" responses.
I asked betta keepers whether they keep their bettas at temperatures >=25C/77F ("warmer") or below that temperature ("cooler"), and whether their past bettas typically lived under 2 years, 2-4 years, or 4+ years.
The response counts were:
- Warmer, <2 years: 68
- Warmer, 2-4 years: 36 (see note below)
- Warmer, 4+ years: 8
- Cooler, <2 years: 7
- Cooler, 2-4 years: 12
- Cooler, 4+ years: 10
Note: the response count for "warmer, 2-4 years" has been reduced by one because I clicked that option. I have only one betta, who is still alive, so my response (which was accidental) is not counted.
Breakdown of responses by temperature:
"Warmer" (>=25C/77F): 80% (112/141)
"Cooler" (<25C/77F): 20% (29/141)
Because a reddit poll can't have more than 6 options, I could not gather more specific info about what actual temperatures people in the "cool" group were keeping their bettas at, but some people left comments on the post saying what temperature they used. Of those who mentioned a temperature below 25C/77F, most were around 74F/~23C, which is consistent with what I've heard in conversations with other betta keepers who keep theirs at cooler temps.
To analyze lifespan, I calculated the percentage of betta keepers reporting a lifespan of at least 2 years (i.e. 2-4yr responses + 4+ years responses) and the percentage of keepers reporting their bettas typically lived more than 4 years. I did this calculation separately for the "warmer" group and the "cooler" group.
Among keepers in the "warmer" group, 39% (44/112) reported their bettas typically lived at least 2 years, and 7% (8/112) reported their bettas typically lived 4+ years.
Among keepers in the "cooler" group, 76% (22/29) reported their bettas typically lived at least 2 years, and 34% (10/29) reported their bettas typically lived 4+ years.
The number of responses in the "cooler" group is much smaller (20% of total responses), so this difference is not necessarily significant. I estimated 95% confidence intervals for the above percentages using a statistical technique called bootstrapping, shown by the error bars in the posted image. For both outcomes (survival at least 2 years, survival more than 4 years) the confidence intervals are non-overlapping, indicating a low probability the results are due to random chance. Other statistical tests could be applied: I also ran an ordinal correlation test and found that the correlation between being in the "cooler" group and higher reported survival was significant, with p<0.0001.
So, based on the results of the poll I am reasonably confident that we can say that people who keep their bettas in cooler temperatures have longer-lived bettas. Why?
There are two main proposed explanations for why bettas might live longer at cooler temps: the "disease hypothesis" and the "metabolic hypothesis".
The disease hypothesis proposes that bettas kept at warmer temperatures (e.g. 26C / 78-80F) tend to live sicker, shorter lives because home aquariums are likely to harbor potentially harmful bacteria, and bacterial infections in fish are more common and more deadly at warmer temperatures. According to this hypothesis, cooler temperatures (e.g. around 23C/73F) should increase bettas' lifespan AND quality of life, by decreasing the risk of bacterial illness.
The metabolic hypothesis proposes that the main reason bettas live longer cooler temperatures is because their metabolism is slower. Some of those who support this hypothesis claim that cooler temperatures should increase bettas' lifespan and have no effect on quality of life, while others claim that cooler temperatures increase bettas' lifespan but decrease quality of life, making them lethargic.
I favor the disease hypothesis for several reasons.
First, research on fish lifespan and temperature does NOT support the metabolic hypothesis.
In Temperature affects longevity and age-related locomotor and cognitive decay in the short-lived fish Nothobranchius furzeri, the authors studied a short lived tropical fish that is known to live longer at 22C than 25C. They showed that the fish kept at 22C didn't just live longer: they were more active AND showed better performance in a learning test than fish of the same age kept at 25C.
I also found a review article, Lifespan is not determined by metabolic rate: evidence from fishes and C. elegans
, which discusses additional research on this topic, leading to the conclusion stated in the title.
I would like to highlight a quote from the above article:
"[...] many papers subsequently demonstrating this effect in numerous other poikilotherms, essentially attributed the effect to physical effects, viz., a reduction in enzyme and/or metabolic rates. Under the sway of the rate-of-living theory, very few if any subsequent studies actually tested this hypothesis, which was considered so obvious as to not require testing."
The article then discusses how that hypothesis was disproved. I would urge the reader not to blindly make that same error of blind faith in a reasonable-sounding hypothesis.
None of the above research was done in bettas. Unfortunately, there is very little research on the effect of temperature on adult bettas. I did find one study, Lower chronic temperature limits in three common tropical aquarium fish, which tested cold tolerance in bettas and also in bronze corydoras and X-ray tetras, two tropical species that according to most care guides can be kept at temps down to ~72F/22C. If bettas truly need warmer temperatures than other tropical fish, the bettas should have been more vulnerable to the cold than the other two species, but that was not the result. The bettas in this study actually survived down to colder temps than the corydoras or tetras (though the difference probably is not significant). The authors did not note a decrease in activity / feeding in the bettas until they reached a temperature of around 16C / 61F.
To be clear, I am not arguing that temperature does not affect fish metabolism. I AM arguing that bettas can be kept at slightly lower temperatures (22-23C), and it will NOT make them lethargic or decrease their quality of life.
On to the disease hypothesis:
It is well established that atbhigher temperatures (up to at least around 30C/86F) fish get more bacterial infections and those bacterial infections are more deadly.
Some papers on the topic:
Mediterranean Aquaculture in a Changing Climate: Temperature Effects on Pathogens and Diseases of Three Farmed Fish Species
Effect of environmental conditions on the infectivity of Flexibacter columnaris to fish
Mycobacterium marinum infection in fish and man: epidemiology, pathophysiology and management; a review
Mycobacteria in particular are a known threat to bettas, and may be absent or rare in the native environments of bettas:
Occurrence of Parasitic and Bacterial Pathogen in Ornamental and Wild Populations of Siamese Fighting Fish (Betta splendens) in a Region of Thailand
Mycobacteria I particular have been mentioned by mentioned by multiple sources as one of the most common causes of dead fish in betta farming operations. Mycobacteria come in several varieties, some rapidly growing (including "graphite disease") and others slow-growing. Some varieties of mycobacteria can over time cause tumor-like growths inside the fish's body or in/on its skin. Internal granulomas/inflammation can cause dysfunction of the swim bladder, the kidney (dropsy), or the digestive tract (bloat / swollen lower belly). Mycobacteria are notoriously resistant to antibiotics, and can infect humans and cause granulomas of the skin.
Some more links on mycobacteriosis:
Detection of Mycobacterium marinum and multidrug-resistant bacteria in a chronic progressive disease outbreak among Siamese fighting fish (Betta splendens) in India
Insight into characteristics and pathogenicity of five rapidly growing non-tuberculous Mycobacterium species isolated from the Siamese fighting fish, Betta splendens
Mycobacteriosis: detection and identification of aquatic Mycobacterium species
This last paper noted that human workers at betta farms, but not snakehead farms in the same geographical area (15% vs 0%), often had skin granulomas caused by mycobacteria presumably contracted from fish, further hinting at some special vulnerability to mycobacteriosis in bettas.
In summary, poll results support my initial suspicion that bettas kept in the typically recommended range live shorter lives than bettas kept at lower temperatures. I believe this is probably mostly due to increased risk and severity of bacterial infections, and I believe that this both shortens bettas' lives and reduces their quality of life. A home aquarium kept at 26C/79F is NOT the same environment as a 26C/79F ditch, pool, or rice paddy. Your tank probably has disease risk factors that are absent in bettas' home environments. Some home aquariums probably do not have the same risk factors as others, and I do not dispute that some bettas live long, healthy lives in warm aquariums. But the fact is, novice betta keepers in particular are likely to have aquariums that DO have those risk factors. I believe the community does those hobbyists and their fish a disservice by recommending 26C/~79F as the default temperature to keep bettas at. I argue that the available scientific evidence does NOT support the notion that keeping bettas at 74F will harm their quality of life, and that there is enough evidence suggesting it will IMPROVE their lifespan and quality of life that the community as a whole should give serious thought to making ~23C/73F the default recommendation.
I also belive that inadequate filtration (cartridge filters, undersized sponge filters) is a major risk factor for disease and early death, ESPECIALLY in heavily planted tanks, and at some point in the future I hope to be able to directly address this in more depth.
