I had the bigger fahaka (about 2'') for weeks, and she is the one keep staying in sand for long time. I just add another small pee fahaka (around 1cm) a few days ago. The pee fahaka is swimming around, and it's like 4 or 5 times smaller than the bigger one. Is it possible that the big fahaka scared? But before adding the pee fahaka, she's already got used to bury in sand since the first time she bury herself.
I will keep my eyes on them, but I dont see they are unhappy with each other. They are just ignored each other lol. I also put lots of fake plants and rocks for hiding. As I said, the pee fahaka will swim to me usually and she doesn't looks scared at all. The problem is the big one seems she doesn't like human. Everytime I walk close to the tank, she's trying to hide behind plants. That's the problem I want to solve.
Thank you all the inputs.