So they get used to the height and lower atmospheric pressure.

So they can acclimatise to the weather

It is the lag phase where an organism try to acclimatise in new environment.

Every fish lives in a different inviroment.If its a tropical fish and the water is to cold it will die.

yes they can, so long as you acclimatise them to the tank. For starters, if possible, fill your tank with water from the pond, ( I know, it probably won't be the clearest, but your fish will thank you), and once your aquarium is up and running, match the temp of the tank as near as possible to the temp in the pond, (ie, don't try and throw them in your tropical tank, they will probably go belly up!). Bag the fish, and acclimatise them to the aquarium as you would with a store bought fish. To get rid of the bugs, dose the tank and the fish with malachite?(been awhile, can't remember the spelling) blue. Hope that is at all helpful