Water level in airlock dropping

I've just read another post about this and it seems that it's not too much to worry about but I checked my beer today and the level of water in the airlock has dropped. I last checked it 2 days ago so I'm wondering what would have happened yesterday to cause this. The beer has been fermenting for a week now. I read an explanation that said this could be down to not cooling the wort down enough before pitching the yeast but I was very careful to get the wort down to room temperature (70F) and I don't know why this would make a difference almost a week later anyway. The fermenter temperature has remained pretty steady between 72 and 74.

I know I need to top it up again so can I just do so with water and star san again? I know the water has nowhere to go but into the bottle so am wary of getting too much star san in it. But I'd also like to know why this is happening if anyone has any ideas!

 

Suck back can often happen due to temperature differences that result in a negative pressure in the fermentor. I would just top up with StarSan, as the volume is so small. However, if you are concerned you can use high proof vodka.

 

Thank you!

 

You don't reveal your location, but if its in the North where the relative humidity is 10% water will evaporate pretty quickly.

 

The Bahamas! The room the beer is in is relatively cool considering and so far I've managed to keep the temp
fairly constant but I'm sure humidity will still be a factor

 

This is pure speculation, but maybe a 3-piece airlock is less likely to experience (as much) fluid loss because in the event of suck-back, the inner dome gets sucked against the bottom of the airlock well?

 

Am using a three piece airlock already...