i`m gonna assume you have 3 lines exiting the tank..and three sizes. the biggest one is the feed to the pump...if you disconnect it at the pump can you siphon from it??? if you can its a pump problem.. if you cant its likely a kinked hose at the top of the tank...or your hoses got swapped around. remember ,the largest line is the feed line, if there swapped around its at the tank. did one hose seem unusually tight putting it on? i`m thinkin a kink somewhere.