temperature change, how dangerous?

Firstly i hope your using a dechlorinator for your tap water. Secondly i would not recommend using rain or pond water as you do not know what nasty things are hiding inside the water from these sources.

As for the question of Temperature shock. ask yourself how many times can run around outside on a rainy winters day befor you get sick?
Exposing your fish to temperature drops or rises is not directly bad for them, but is stresses their bodies and this could compromises their immune system making them susceptible to other diseases such as ich.

Remember keeping your fishes aquarium as close to their natural environment will help keep them as healthy as possible.

 

With rain and pond water there might not be chlorine but there might be other bad things in there.... acid rain and such.
As for adding tap water straight from the tap how can you grantee that the water parameters are what you need in the tank, ie pH gH temperature chlorine, chloromine, nitrates etc..
Is it not easier to put it in a bucket and make sure its alright for your fish?
its easier to treat water befor it enters your closed ecosystem in your tank than trying to fix a problem once it is inside your tank.
Yes in nature there are thing like temperature fluctuations but there is also things like stability and fresh flowing water keeping things like nitrates stable. Things we can't emulate in an aquarium like constant supply of fresh new water from up stream.
As for people not having bad experiences with putting tap water straight into their aquarium's. If you only changing 10% of your water then no it wont fluctuate so much. but lets say you keep discus fish which require you to keep water from about @ 27 - 30 and its winter at you do a 25% water change. this might lower the temperature to much and stress your fish, making them susceptible to disease. and thats what you need to avoid.

I'm not saying it will happen only that it might happen. personally I prefer to do as much as possible to keep my fish in tiptop shape. stuff like treating for chlorine befor i add the water to my tank. heating the water if i have to do a water change of more than 20%, These precautions might not be necessary but they provide a safety net if you will.

If you feel that what your doing is the right thing for your aquarium thats great it is after all your live stock and your money.

 

how much water do you change?

 

here is a good article on tempreture change and its effects on fish
http://books.google.co.za/books

 

sorry try this one
http://books.google.co.za/books?id=SAdDtT7YRRoC&pg=PA60&lpg=PA60&dq=water+temperature+changes+in+an+aquarium&source=bl&ots=5zSR-36LWo&sig=JMIshJ6IMqvAcyBiEIYVRIMwiu8&hl=en&ei=FiT5SuqXGIzVngf1p_SDDQ&sa=X&oi=book_result&ct=result&resnum=10&ved=0CBoQ6AEwCTgo#v=onepage&q=water%20temperature%20changes%20in%20an%20aquarium&f=false

 

not to sure about this but in my opinion i think it would be a bad thing. i mean unless you can controll the amount they are exposed to how do you know what your exposing them to is not leathal?