I had this same problem a couple of years ago and almost broke down the tank because of it.Tried several powders,Sevin Dust was one and nothing worked.Then i got a hold of a spray called"The Enforcer".It worked wonders.Comes in a one gallon container with a spray nozzle.It is good because you carry it and can spray however how you want to.I turned off all fans and a/c.One treatment was all it took and i did not bother to cover anything on the tank.I just sprayed real low to the floor.You do need to be able to leave the house empty for one hour after treating.It is also for use on furntiure and bedding.Here is a link,it is the gallon size container kinda in the middle of the page.

http://www.enforcer.com/pages/fleahouse.html