As long as you are within a few degrees you are fine. Being off by 3 degrees changing 1/3 of the water will create a 1 degree temp drop, something that does occur regularly in nature.
If your tanks are around 80F, and your cold tap water is around 50F in the winter, you do need to heat it. Changing 1/3 in that situation will drop the tank to 70F, changing 2/3 will drop it to 60F, if you change it quickly. Trickle it in with a submersable heater, & the difference will be much less.
I change between 50% & 80% on my tanks, within 2 or 3 degrees F causes no problems.
If your tanks are around 80F, and your cold tap water is around 50F in the winter, you do need to heat it. Changing 1/3 in that situation will drop the tank to 70F, changing 2/3 will drop it to 60F, if you change it quickly. Trickle it in with a submersable heater, & the difference will be much less.
I change between 50% & 80% on my tanks, within 2 or 3 degrees F causes no problems.
