Does the cooling system have to be bled after replacing coolant expansion tank ?

[Fastlife91]

I replaced the expansion tank as old one was leaking.

Since then I notice every 2 days or so the water level drops very slightly. can't see any visible leaks, no sign of HG failure , but I just wondered if it was possible that there may be air in the cooling systems after changing the bottle, if so how does that cause coolant loss and how should the cooling system be bled , ???

 

thanks