Welcome to ECF. You could try running the tank under hot water in case e-liquid has gotten in a nook and cranny somewhere, that may help loosen things up. If that doesn't work you can use rubber gloves or a rubber mat to try and gain more leverage, placing something as a counterpoint in the airholes may work too. You may want to invest in a "mini gripper" which opens and loosens tanks in the same way a larger one loosens stubborn jars, and etc. I know
fasttech sells them as do many other places.
Best of luck and in the future, make sure you aren't overtightening things.. Finger tight is usually enough.
Anna