Whenever I go collect my fish from whatever body of water (lake, river, or pond) I bring at least a three gallon cooler. One that holds like a 12 pack. This is so when you get to the body of water, you can keep the water for your fish the same temperature as the water the fish are coming from. Then I bring them back to the house and open the cooler so the water becomes room temperature. My native aquariums are room temperature so this will slowly get the new fish used to the aquarium temperature. After about thirty min I add a gallon of aquarium water to the five gallon bucket. Then I wait another fifteen minutes or so and add the fish to the tank. It's not an exact science but that has worked for me.