Pea puffers acting strange??

Hey, I've had my pea puffers for about a week and a half now. All of them are juveniles and quite little. I have 6 of them in a 20 gallon tank. Everything was fine with the water etc. However, I went in to go feed them today and they were all at the top of the water almost looking like other fish when they eat. I assumed they were hungry and looking for food so I fed them, I feed them frozen bloodworms brine shrimp and small snails for now. And none of them went to go eat. So I thought this was strange to begin with. Then 2 of them starting hopping across the surface of the water and then would go belly up, they are still alive but they would just sit there belly up and then flip back over and freak out.
Can anyone help me out on figuring out what's wrong??
Thanks in advance!