mitai May2, originally uploaded by misa212.

Q: LAB wrote,

I’ve been looking through your blog – so much helpful information. I’m currently cycling a tank (without fish), and am trying to regulate the temperature. During the day, I’ve got a light on for the live plants, and the water gets to about 80-82 degrees F, and at night, it gets cooler, so I’ve got a heater set to 80 degrees F. I was wondering how much of a temperature fluctuation the Bettas can handle safely? The time I’m most worried about is during the day, when the light is on.I’ve got a 3 gallon aquarium, (the Eclipse system).

Thanks for your great blog and website!

A: It’s true that rapid water temperature changes can lead to stress in aquarium fish and even thermal shock if extreme enough but the exact number of degrees that will cause damage can be difficult to pin point.

Water temperature fluctuations generally become a problem when they

1. Are drastic (alter by several degrees)
2. Are prolonged (fish that are kept at temperatures too warm or too cold on a regular basis)
3. Fluctuate over a short period of time

These temperature changes also effect the fish differently. A drastic temperature change can cause an immediate physical or behavioral change in the Betta. It may swim erratically, float on one side or appear distressed. Less severe fluctuations of just a few degrees over time may effect the fish in a less obvious way. For instance, the immune system can be weakened leaving the Betta prone to illnesses like bacterial infections or parasites. In these cases it may not be obvious to the fish keeper that the cause was stress induced by temperature changes.

The general health of the fish also comes into play. Weak or sick fish are more likely to feel the effects of temperature fluctuations. This is one reason why breeders won’t ship fish that aren’t in perfect health as shipping can often expose them to fluctuations of 10 Fahrenheit degrees or more.

That said, I personally try to keep my temperature fluctuations less than 2-3 degrees Fahrenheit in a 24 hour period (or 1-2 degrees Celsius.)In the summer this can be a bit more difficult so I try my best to keep my fish healthy by feeding them a balanced nutritious diet and keeping their water very clean.

One thing you can do is reduce the amount of time you keep the light on over your tank, particularly in the warmer months. As long as he’s getting ambient light he’ll be fine. The tank light is really more for your benefit than the Betta, unless you are using it to grow live plants that require a lot of light.

Aim for a temperature of 78-80F. A few degrees above or below that is fine, but of course best if you can keep it stable.

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Reader Interactions


  1. Bhuvana says:

    It’s true that temperature fluctuations for betta fish should be done with a lot of care and attention.
    My betta fish died due to severe stress casued by a mild temperature fluctuations and I feel terrible bout it.
    I got him the pet store, he was in such a small cup and I gave him so much space by tranfering her into a big 2 gallon bowl. He was very happy and would often come close to me and wiggle and play.
    I used to play with him everyday and he was highly reciprocative.
    I had him for 7 weeks and every week I would change him water every week.
    This week when I moved him into warm water I thought he was very excited but that was severe stress caused due to immediate fluctuation in water temp.
    One small inaccuracy in water temperature caused his death…

  2. Anonymous says:

    Oh gosh, all this is scaring me a bit. I just got my betta today, and burst into tears when I saw that the temperature of his water was 84 degrees F. It was 88 degrees just a few hours earlier, and I don’t know what to do! It’s almost 11 here on a school night and my mother wont let me change the thermostat. Will his immune system be weakened at best if the temperature stays at 84 during the night? What about every night? Help please!

    • LOL says:

      You’re an idiot. if the temp in the room is 88 then the temp in the tank is probably 65 ….. if the tank is 88 then your room is probably 115 degrees LoL

      • sw says:

        She said “temperature of his WATER”. Not her room temp.

      • Chandell says:

        Just reading this now as I am having difficulty with my heater as well. That remark you made “you are an idiot” was so uncalled for. Not everyone is as knowledgeable when it comes to fish keeping especially when first starting out. People come in here for help and knowledge. Keep your nasty remarks to yourself!

    • Rich Richy says:

      Both those temps are fine for your beta FYI

  3. Captain Shnappie says:

    My betta suffered a similar situation today. I’ve had the little guy for just 3 days, keeping the tank temp at a constant 80. Just a bit ago I took a look at the thermostat and it had spiked to 88 for some reason. Ouch. I quickly unplugged the bottom heater and exchanged cup after cup of unheated water at a time to gradually bring the overall temperature down without causing shock. My betta’s heat distress signals were, as stated above, erratic energetic swimming behavior coupled with sideways swimming. Fortunately he is looking better now! I hope my heater is done playing tricks.

  4. Gillian708 says:

    I bought a digital thermometer for $8 on Amazon that is SO much more accurate than those stickers. I though my temperature was 80 when it was truly 85. I think the stickers must be affected by room temperature as well. Search for “digital instant read thermometer”

  5. Taylor says:

    My heater quit working and when I just went to feed my Betta the water was it a chilling 64 degrees. I switched tank heaters and the heater I put in is working. Will this harm my betta to be in the tank as it warms back up?!

  6. Songwinged says:

    Help! I keep my betta fish downstairs, where the temperature is often 65 degrees because we have to put ice cod temperatures so heat can go upstairs (long story). I put my finger in the water when I was changing it and it was ice cold. My betta was active though but not as active. I don’t have a thermometer and the tank is so small we cants fit a water heater. What should I do?

    • Rohit says:

      Go for a larger tank. 10G is better. Large volume of water are not affected drastically by temperature fluctuations as compared to small bowls of 1~5 G. (Temp changes slowly in larger volume).
      And since it is so cold, go for a heater and thermometer. Until you get this in order, move the tank in warmer area (upstairs)

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