Polka dot catfish is a common name for synodontis angelicus which is not a brackish species. There is a bit of information about them here under the fish from Zaire, about the middle of the page, http://www.fishchannel.com/freshwate...catfishes.aspx

I am sorry to hear you found your fish caught in the propeller thing, a power head maybe? Was this the puffer or the catfish? Healthy fish do not get caught in power heads or filters. Maybe with some information from you we can help determine why it got stuck. Tell us how you cycled your tank, with fish or without, how often and how much water do you change, how do you clean the filters and how often, and finally can you give us the current water parameters (ammonia, nitrite, and nitrate levels)?