then just drop in your heater and monitor temp. if you dump warm water in you will be spiking the temp every time you do and adding stress to the the already stressed fish.
i third the coolers . traveled with 25 fish i bought in chicago all the way to sw corner of kansas (16 hr drive ) and water didn't drop but 3 degrees from the tank tempered water they were in from start to finish . i did however use a battery powered bubbler to supply some oxygen .