Avocado leaves in bad state

[gergana_balabanova]

Hello everyone,
I have a few avocado plants that live indoor and the leaves of most of them turn brown and curl up, beginning from the tip, and after that fall off. I spray them twice a day with water and there is a humidifier working in the room. I thought the leaves turn like that because of the dry air but now I am wondering if there are some pests. I would be happy if someone could suggest what the problem might be.

 

[Chuck]

That is from the soil being kept too wet.