Mollies behave different in different tanks
I have a 55 gallon tank with many fish in them, including about 22 nearly full grown to full grown mollies, a 6 or 7 inch crawfish, a 5 or 6 inch convict chilclid, and an 8 or 9 inch pleco. This tank has fake plants and rocks along the back portion and a long clear area near the front.
I have a 20 gallon tank with about 30 some odd mollies that are about a half inch long (they were born in July and September). This tank has live plants spread all over the bottom with leaves and stems touching the top. This tank also has one 2 inch algea eater, one 4 inch blue powder dwarf gourami, one black neon tetra, and one guppy.
In the 55 gallon tank my mollies tend to stay semi motionless near the top of the tank. In my 20 gallon tank I find most of the mollies motionless on the bottom of the tank when I turn the lights on.
Do mollies prefer to rest on the bottom of the tank or the top? Does it matter? I believe the reason the mollies don't sleep near the bottom is because the pleco likes to swim the length of the front portion near the bottom as he hunts for food. Also, they may not like the idea of that crawfish who comes out at night to scavenge. However, this has not stopped the red wag swordtail from resting on the bottom. The convict chiclid also rests on the bottom, but I doubt he really fears any fish in the tank.
I could also mention the new born mollies I have in my 10 gallon tank. They were born sometime between midnight and 6 AM. I found them among the plants, laying flat on the bottom, and between the glass and the breeder container. But I am pretty sure their behavior is due to their survival instinct to hide from their mom. I promptly moved the mother fish into the breeder with the other pregnant female molly and then moved as many of the babies as possible to the other side of my 10 gallon tank (I have divided it in half). They are now with my red wag swordtail fry who are about the same size. I moved 24 mollies and I know there are more, but I had to take my oldest son to school and go to work. My wife will move the rest some time today.
Robert W. Marda