It's generally accepted that it's good to get the temperature of your new water to match the temperature of the tank water before introducing it. You don't want to change the temperature of the aquarium too much when you add the new water, or you risk a temperature shock that can kill fish.

Are there quantitative recommendations as to how much the temperature of the water can vary before and after the water change?

My own research shows plenty of individuals declaring "0 temperature change is the only acceptable answer." Others argue that a sudden rainstorm drops the local temperature of water, so fish are used to some change. The aquarium wiki has an uncited claim that you want to avoid instantaneous changes greater than 5 °C (8 °F)

Are there any studies which have shown quantitatively what the effects of instantaneous thermal changes on a reasonable order of magnitude have (I'm thinking like 0 °C to 8 °C changes). I've seen studies which covered the effect of 20 °C changes, which are obviously dramatic, but nothing on the smaller changes.

I'm particularly looking at freshwater tanks, but saltwater numbers may be useful for others.

  • I won't answer this formally. I've had emergencies due to heater failure where I put fish through a 20F+ degree temperature change instantly and they were fine. I don't think you'll find a lot of quantitative studies on this because it's ethically questionable and a best practices like equalize as best as possible is technically sufficient for the health of most fish.
    – Jestep
    Feb 1, 2019 at 21:04
  • @Jestep I did find studies which subjected fish to rapid transitions from 75F to 40F, so I'm not sure if the ethics have stopped the scientists. I just can't find anything less extreme.
    – Cort Ammon
    Feb 2, 2019 at 4:12
  • Answers will depend on the type of fish. The effects can also be psychological in addition to physical: it could cause the fishes to panic or the usual "become stressed" which makes them vulnerable to other problems. Temperature variation will also affect the bacterias present in the water, which multiply faster when warmer.
    – Manuki
    Apr 24, 2019 at 18:27

2 Answers 2


I manage to keep mine to 1-2 °C temperature change when doing up to 50% water changes. That is, the new water is warmer than the existing, and I add cold until it's at least within 1-2 °C, normally and then siphon it into the tank.

I can get it down to 0.5 °C without too much difficulty. I'm unsure why you'd subject a fish to anything more than this, to be honest.

I set up a new tank for my daughter about a week ago, for fish that were listed as being in a temperate tank at the store (with the same fish also in a tropical tank else where in the store). The store couldn't give me an accurate temperature at the time, somewhere around 25 °C they said. We had them in a tank at home around 24 °C for two days, before I started bringing it up to the 26 °C that I tend to keep tanks around.

Increasing just ~0.2 °C, a day, and we ended up losing half the fish anyway. It may of been for other reasons, but I'm sure the slow temperature change still upset them somewhat.

  • 1
    I highly doubt that a 0.2C increase per day had anything to do with your fish dying. Most likely just had to do with sickness, or your tank not fully cycled yet. I'm doing 40% water changes (so probably around 60litres), and the temperature probably fluctuates 2°C. Never had a dead fish because of that.
    – Diether
    Feb 8, 2019 at 14:27
  • 0.2C a day is nothing. I expect that the water in my aquarium has greater temperature fluctuations daily by itself - just room temperature, no need for heater most of the year. Verify the quality of the water before other things. Considering that it is a new aquarium, I suspect an incomplete cycling.
    – virolino
    Mar 15, 2023 at 6:39

I use my thermometer in the tap until it's within 1 degrees of the tank water. If your doing a 50% water change and your tap is 1 degrees different, when it blends with the tank water it would only be around 0.5 degrees different. Not a big deal. If you use a heater in your tank it will swing at least that much anyways before it turns on to warm it up again

  • 1
    Welcome to pets.SE! Would you maybe share some information about your tank? I can imagine that different size, different kind of fish tolerate a different amount of "failure", right? Mar 14, 2023 at 16:35
  • That's an excellent point about the thermostat hysteresis that I did not consider
    – Cort Ammon
    Mar 14, 2023 at 17:04

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