recently my yellow molly has been sitting at the bottom of the tank only to come up for food. and it doesn't look like it swims too well.it has gotten paler and it's back seems to arch more. I have 6 fish in total and a 10 gallon tank. there is another yellow molly that sometimes chases it and it has been doing that ever since I got them. please tell me what's wrong and how to cure it. thanks!