FINCHES SUDDENLY DYING!?

I've lost a Cordon Blue male and Zebra male in one week. Very healthy. Seemed fine night before til suddenly they were at the bottom of the cage nearly dead in morning. a nightlight is kept on. In a new cage that's a bit smaller than the last aviary. Gouldians in same cage had airsac mites but cured. No symptoms on these two. Thoughts please??