It is possibly not because of the tap water. That would cause mineral build-up in the tips and edges of the leaves which would lead to crispiness in those areas not yellowing in general.
Low humidity would cause similar symptoms as tap water, crispiness instead of overall yellowing.
Based on that you have upped the watering I would suspect root rot. You could try to remove the plant from the pot without disturbing the root ball too much to see if there is any rotted roots visible.
If there is rot you can try to repot the plant (even if it is winter) into a drier substrate. Make sure to gently clean the roots and cut off any rotted parts.
If you would not like to do such a drastic measure as repotting the plant, you could use diluted Hydrogen Peroxide to treat it.
From How To Use Hydrogen Peroxide For Root Rot?:
Root rot is most commonly caused by poor soil aeration or over
Mix one part 3% percent hydrogen peroxide with two parts water and
carefully pour it over the plant’s root system with a watering can or
This will kill off the bacteria which causes root rot.
Additionally, hydrogen peroxide will break down once absorbed in the
soil, releasing extra oxygen and encouraging root growth by creating
more space in the soil.
You may use this same mixture to disinfect potting mix or soil prior
If you do not repot make sure to gently loosen up the top of the soil with a chopstick or other tool to increase the aeration of the soil so it will dry faster. You can also consider moving the plant to a place where the soil could dry a little faster in general.