It's the colder weather. Plants would decrease evap.
Warmer air can hold more water... This, "relative humidity."
Lets say the humidity outside is 60%.
That air, and all the water it's holding comes inside and gets warmed up. That relative humidity drops. Meaning, it can take on more water easily.
The humidity in your home has remained stable by taking more water out of your fish tank.
Maybe you've noticed, though house plants grow less in winter, they need more frequent watering for the same reason as your tank levels are going down faster.