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Temperature difference of water, how much is too much? Are fish really that delicate?


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We see everywhere how you have to acclimate fish to the temperature of your tank by flaoting the bag, and how they can be shocked and die with sudden changes in temperature, but my question is, how much difference is needed to shock a fish? I guess it's different from the type of fish but any general idea? 3, 4, 5 degree difference? Or are we talking 10+?

I wonder this because I do not remember where I heard it, but someone said fish in the wild are always moving from one pocket of temperature to the next one, let's say they swim arround in an area of the river where direct sunlight is shining on it, temperature there might be 75F-79F, but then a fish might get scared, or it might just want to relax and he will go below some rocks, a sunken peice of driftwood, or perhaps a natural little cave, where the sun barely reaches, and the temperature is way cooler, perhaps 70F-75F? Can you imagine fish dying left and right in the wild due to temperature shock?

I've made little experiments with my molly tank, and have made water changes with increasing number of difference in the water. I started with 2 degree difference, then 3, then 4... today I did a water change of 50% with water that was about 87F. It mixed with the other 50% in the tank that was at 79F, and the final temperature was about 84F. I have never seen signs of stress or have lost a fish, not even fry. Not even discolored due to stress, however I am afraid to go any further, I do not want to abuse my pets, certainly wouldn't do it in the blazing heat of the summer days in here, where tap water comes out at 100F+.

So what's the deal, has anyone made such an experiment? How much of a temp. difference can a fish take?

Edited by HenryC
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I don't know that there is a true answer, and I certainly don't have one. I think in a health established tank, fish can probably handle more swings. Think about folks trying to encourage certain corys and plecos to breed, they will do some fairly significant temp swings. 

I think the main concern is when brining in new fish from a local store, hobbyist, or through the mail. I think in these situations it is more to do with the accumulated stress. Knowing the water parameters may be different, maybe they've been the mail for two days, maybe they have an illness, this is where I think the temp acclimation is important, it removes one stressor. 

When I do water changes, I am simply going for similar, I know it is never perfect.

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