My platies don't hang out at the bootom of the tank. At any given time, if I want to find them, they are hanging out behind the lip of the filter. The are almost always there. At first I worried about it but then I decided that they just liked it there. They eat fine and I haven't noticed any other problems with them. The females are huge and have been for about two weeks. They look like they are about to explode! They are twice the size of the male. He looks like fry compared to them.
My betta likes to "sleep" on the bottom the the tank. He curls up next to the bubble net so that he always has fresh air. He will lay there for hours! Then he just suddenly decides to attack everything in his tank and swim around for a couple of hours. He shows no signs of illness, so I just let him be.