A sudden change in temp of 3-4 degree's is actually enough to cause some fish to have dropsy, some fish cope with day and night fluctuations in temp a lot better than others
But we're not talking about a sudden change, no ones coming along and dropping a lump of ice in the tank and unless you live in a wooden hut in Alaska your house temperature isnt going to drop by 4 degrees the moment the heating switches off, if it does then i seriously suggest installing insulation and double glazed windows.
The temperature range in a single body of water at one time can vary greatly at different depths with the surface of the water actually being up to 5 or 6 degrees warmer than the water at the bottom, are you telling me that nature created animals that will become sick and die if they swim upwards or downwards? I have taken temperature readings from varying levels and distances from the bank in lakes and rivers while fishing (knowing the temperature lets you know where the fish are likely to be and how hungry they might be) and have been astounded at how much difference there can be in a few meters of water.
Well when i was living at my old flat and we had the powercut one of my tanks dropped from 25 degree's to 22-21degree's in less than 24hrs, i would call that a large sudden change in temp. Although it did cause the danio's in the tank i had at the time to spawn, most other fish would not fair well with such a large change in temp in a single day (the chinese algae eater i had at the time looked very pale and listless, obviously quite stressed from the change in temp).
Also, generally speaking, all coldwater fish i know of are a lot hardier than most tropical fish- take the common goldfish for example, it can live in temps anywhere from below freezing to a toasty 30 degree's C warm (although it probably wouldn't thrive at such a warm temp, it would survive it relatively easily though if there was enough oxygen in the water and the hot temps were not prolonged for too much in the long term)- a betta on the other hand would no where near survive that greater range of temps though, and i am sure if i dropped the temp in a betta's tank 4-5 degree's in a single day it would probably put a great amount of stress on the fishes body.
With temp toleration, yes a lot of fish can survive in a great range of temps, but i think it would be better to heat a tank than to let it fluctuate loads day and night, better to be safe than sorry as they say.
I've been to Singapore before and every time i go there it is always very hot and humid, the temperature and humidity levels hardly change at all through night and day, and i've heard that Thailand has a similar climate. Thus i wouldn't be suprised if betta's had adapted to a warm climate whose temperature stayed pretty constant and hot for most of the year around. Fish living in our seasonal climate though are probably a lot more adapted to the massively varying temperatures that we experience during the seasons.
In the wild, you will rarely see fish living in (let alone thriving in) very shallow waters unless it is a very hot country which is experiencing a dry season
How shallow are you thinking?
In any body of fresh water you will find many fish, large and small, in water 10 to 18 inches deep. The shallow water is warmer and gets greater light penetration here so food is more pleantifull and over hanging plants and marginal vegetation offer protection from predatory fish and birds, at further depths you will find less fish and in very deep waters you will find that the majority of the fish will be swimming at no more than a few feet from the surface. Again as a serious competition angler one of the methods myself and other anglers employ is to sit back well away from the edge of the water and actually fish right against the bank close to over hanging vegetation in water that is often no more than 10 inches deep.
People give fish a lot less credit for their resilience than they deserve, yes there are some very sensative species which in an aquarium situation will die in anything less than perfect conditions (discus come to mind) but luckily that is not the norm and most commonly available species including Bettas are a lot hardier than some people seem to think. It is only when you have studied fish in the wild that you can really understand how they interact with their enviroment and cope with the changes it goes through.
Yes but you are talking about ponds/lakes/rivers etc which have shallow area's but also have deeper area's in them, in the wild you simply would not find fish living in a pond whose deepest depth was 10-18inches deep since it would simply practically freeze solid or dry out every year and kill off all the fish. The main reason why you will see more fish spending time in the shallow area's during the day time is simply because they are there to hunt the insect and amphibian life which thrives most in shallow waters (which heat up quicker during the day time, so are good for developing bug larvae etc). Shallow waters are not actually the most protected from birds and predatory fish, they are actually the riskiest places for the fish to visit, since the shallower the water the easier they can get picked off by predatory birds like herons or animals like cats, while at the edges of the habitat the fish are more likely to get easily corned and captured by predatory fish etc- but the fish takes these risks to feed on the prey that generally thrives best in the warmer shallower waters, if they stayed in the safer depths all the time they would get half as much to eat etc.
During the winter time most fish spend a greater amount of time in the deeper area's of their habitat since there is less reason to go to the colder and less fruitful shallow waters in wintertime. I remember with my mothers goldfish pond, during the summer the goldfish would be very active in the shallow area's of the pond, but during the wintertime they would just dwell in a dormant like state at the bottom, since deeper waters are more protected from sudden changes in temp- i guess this must also mean that the goldfish don't like daily fluctuations in their pond temp, since if they didn't mind them then they'd spend more time at the surface of the waters.
I think at the end of the day, i think what temps a fish can survive or thrive in depends a lot on its natural habitat. Back at my old flat, fish like my danio's thrived and spawned when the temps suddenly dropped, because for them they must have thought that the rains had arrived with lots of fresh cold mountain water, but fish like my CAE visibly suffered though, i assume because they come from a habitat that is a lot more stable in temp.
/www.fbas.co.uk/Siamese.html
he's catching wild Bettas from in the plants and roots!
