Polly scale offers an Airbrush thinner for their Acrylic paint. That is it in your post, the blue stuff.
That is what I'm using. I've been airbrushing for about 20 yrs now and I've found that if a manufacturer offers an airbrush thinner for their paints, it usually flows better and mixes better than anything else. I've learned the hard way.
You can also use water, but like the bottles say, use distilled water. Just so none of the minerals in tap watter affect the outcome of your paint job 
Hope this helps 
Thanks,
Robert E.