If your tank lines to pump are clear check to see if water movement .............clean inline filter........ 3 way tap may be in wrong position or sucking air, pull off tank and pump line and using a joiner (50c from hardware 10 or 12mm )connect them together, if pumps then draws water the 3 way s stuffed..... if clear lines from tanks you can see if air bubbles are coming from an air leak ............................. you can have an air leak but water won't leak from it..................... to check for blockage start at tank, pull line off if water flows out with some force its good, if t just slops out then something is blocking the outlet, if it flows okay flush lines to make sure nothing blocking, small bits of plastic can cause big blockages around connectors, depending on pump there may be small mesh screens at entry point...