My 7 yr old son has a freshwater tank, which he originally filled with about 5 chiclids. I'm not sure of the exact type of chiclid, but they were orange, blue and yellow. They were constantly picking on each other, and now there is only one. A blue one. The last orange one died last week. The lone survivor is the largest one of the bunch. Now that he / she is alone, the fish is piling up gravel just outside of it's hiding place (a spongebob pineapple house). Why is the fish doing this, when none of them ever did it before?