Need help on dwarf puffer fish!!!?

I just got this puffer fish for free from the pet store. They said its not looking well and it might die or something. They gave me some brine shrimps for it to eat. Right now its in my 5 gallon fish tank with no other fish in it. There's only rocks in there and right now its on the floor staying still. I left some frozen blood worms with it. What should i do now?