Hi , i have outback 2004 3.0 with problem with the low water level in my radiator it needs to be topped up or it 

fills my expension bottle. i changed the thermostat as the old one was siezed.

should i change the water pump and all the hoses or the pipes from the heater matrix.

p[lease can you help as my subaru dealer cant help me much !!!!!

thanks lennie

Is the car temperature within normal range/s when this happens? If the temp gauge isn't consistently at the "9 oclock" position when running, you may have bigger problems. Is the thermostat you installed oem (Subaru) or generic? When you fill the radiator do you burp the cooling system to ensure no trapped air remains as these cars can be tricky in this regard.

+1 on OEM thermostats only, all we use as aftermarket can be nothing but problems from our experience .
Was the coolant system a muddy mess when pulled the thermostat ?
If you unlucky you got bigger issues than thermostat/ water pump (water pump failure pretty much unheard of but you can check the weep hole to see if pump seal leaking) .
Best way forward here is system/rad flush, refill and run then do a coolant pressure test see system holding pressure, chemical or exhaust gas tester can be used if no other problems get found to see if got combustion gasses entering coolant system (head gasket failure)