How do sudden temperature changes affect fish and what about fish that are in cooler or warmer temperatures than they are used to?

For example, in winter, if the tank heater dies it can sometimes go unnoticed by the owner and tropical fish are left in water about 14 to 17 degrees celcious. The fish don't die, but how does it affect them long term or short term?

Do you mean this particularly towards discus, or towards fish in general?? (You posted in the discus section)

Discus, from what I understand, are pretty sensitive so I don't think a big temp drop or increase would sit well with them.

Fish in general...I've twice forgotten to plug my heater back in and the temp dropped. Thankfully neither time was there a large temp drop and the fish seemed unaffected. However over a longer period of time a fish who thrives in warm water would be negatively affected by being in lower temps...betta's for instance are often thought to live in cold water, however they are the opposite, liking their water in 84+ degrees. A betta in cooler water will be lethargic and inactive while one in correct water temps will thrive and be very active.

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"Being ignorant is not so much a Shame, as being unwilling to learn".-- Benjamin Franklin

My recent experience with fish in cold water wasn't with discus but just some neons and danios. We lost power for two days during a snow and ice storm. Normally I keep the tank at 78ish but during the outage the tank got down to around 60. I peeked in once with a flashlight and noticed one of the neons was swimming kinda funny, leaning to one side. I thought I would lose all the neons! When the power finally came back on and the tank warmed back up, I made a count of all the fish, snails, and my two ghost shrimp. Everyone had survived!! I don't know if I was just lucky or if they can normally survive two days at that temp. I felt so bad too. I had just gotten the neons over ich and then the power goes out and they freeze their little fins off. But everyone is happy and swimming normally now.

Don't know if this helps answer your question or not tho. Just wanted to share my experience!

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Manda in Auburn, WA

29 gallon freshwater community tank. Lots of plants,10 Zebra Danios, 6 neons, 2 ghost shrimp, 1 mystery snail, and 1 trapdoor snail.
<---- Click to help me grow!!!

Nothing happens...maybe ich would start coming up...but mother nature sometimes forgets to stay steady at a certain temperature in the wild. Fish will survive anything. With discus, much more sensitive fish, I bet they won't last... But most fish would. First signs of really cold water is darting across the tank.

I personally haven't had terrible problems with low temps (sometimes unfortunately low), but that's probably from having a fairly tough fish (betta) and cutting the amount of food in half to keep the digestive system healthy. Needless to say, Mr. Betta was HAPPY when I got a heater

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Lead me not to temptation...I can make it there myself just fine, thank you

Sudden changes can stress them and bring on a disease outbreak. They are tougher than we are often led to believe but they should not stay below the mid-80s or above 90 for extended periods of time.

My wfe wanted to add, "they crack out a blanket like their mommie!" I haven't seen that but if I do I'll post a picture.

I've had a lot of experience with this, sadly. I had a 30 tank fishroom hit by an 8 day Canadian winter power loss.
They will survive the intitial drop of about 6c. The shock causes problems, which show through parasites like ich if they are present (I didn't have this problem) or a loss of balance. The fish begin to settle on the bottom, and as it falls, so do they. By 12c, your tropicals are dead. The last to die are the fish from moving water. My tanks dropped to 6c in that icestorm, and even the plants died, however, 9-18 months later, the Cryptocorynes returned.
Since the fish don't supply their own heat, they have obvious digestive problems - I assume they lose gut bacteria which must need warmth.

Too high a temp will first eliminate reproduction in many species. They will gasp at the surface as oxygen levels get below their comfort zone, and cooler water species will perish.

Metabolically speaking their blood flow slows with the lowering of temperature, depending of the type of fish and what climate it is adapted to, it will either cause problems or the fish will be fine. Most aquarium fish will likely get sick as a result of any rapid environmental change. Fish that might be alright are those that live in seasonal climates in the wild; however, too quick of a change could harm them as well.

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-- Within 3 hours they can train a man to stand at the side of a pool and feed them fish!