I've picked up a Portuguese Man of War from the beach and stuck it in my 30 gallon salt water tank for a closer look. These jellies wash up on Hollywood Beach all the time. So I figured, why not. I've kept two of them for 3 days and fed them some frozen mysid shrimp. They water got very funky after the second day. Porbably due to me only using a sponge filter. But I think it would be possible to keep them long term in aquriums. I would have tried longer, but I had to leave to go back to Europe. So I put them in the trash))