Make sure to pick up Subaru "Super Coolant" it's blue. Mixing different types of coolant will shorten the service life of your coolant(the blue coolant supposed to be replaced every 100,000 miles compared to conventional 30,000 miles).
Also wait till your car cools (so first thing in the morning) before filling the expansion tank. If the tank is empty then fill from the top reservoir first(closest to the turbo) until full. Then fill expansion tank to full. check the expansion tank the following morning. If the coolant level is significantly lower than full then It's a possibility that there is a leak.