abf expansion bottle water level

I recently checked my coolant level and the bottle was dry… so I topped it up to min (car was idle + cold) and I've noticed it's effected the idle.

When the car warms up its noticeably jumpy in gear and it often idles around 1500 as if the maf  is trying to find its way.

There are no other mods or changes I've made a part from this and checking the  fluid again the bottle is dry… oil level is ok and no noticeable leaks. 

Any ideas? I'm not sure the concentration of coolant as when I top it up it appears clear. (Less than 200 ml I put in every time) 

Should I be topping up the coolant with the engine running ? How would an air lock present?

top it up engine off, the system should self bleed fine especially if you're only topping it up but check the thin hose on the tank is flowign coolant back into the tank when you rev it up. if you're using the abf top rad hose the thin pipe is prone to clogging up as for some reason VW put a plastic insert in the connection with a really tiny hole thru it.