I have a 50 gallon tank for a turtle. I am using a H.O.T. magnum filter that works well. I am trying to set up a second filter, also H.O.T. magnum. I cleaned it out, replaced a couple of parts but can't get it to run. It runs for about 5 minutes and starts coughing an sputtering. I suspect it is leaking air somewhere but haven't found any obvious leaks. Any suggestions?