Water movement is much more effective than plants for oxygenation of water. Your live bearers will over populate a small pond in less than a season. Water movement can overcome the negative affect of the organic decomposition  . I just cleaned my small pond ( 10' X 5' X 3'deep) , after a few years it had nearly a foot of needles, leaves, pine cones , branches. etc. The fish were fine in that mess ( about 100 giant danios and 2 large Koi ) because of water movement. So I think your fish ran out of oxygen because of overcrowding and decomposition of plant material. An air pump/and/or a filter pump is needed.

Water movement is much more effective than plants for the oxygenation of water. Your live bearers will overpopulate a small pond in less than a season. Water movement can overcome the negative affect of the organic decomposition. 

I just cleaned my small pond, which is has a surface area of 10' x 5' and 3' deep (3 x 1.5 m on surface and 91 cm in depth) after a few years it had nearly a foot (30 cm) of needles, leaves, pine cones , branches. etc. The fish were fine in that mess (about 100 giant danios and 2 large koi) because of water movement. 

So I think your fish ran out of oxygen because of overcrowding and decomposition of plant material. An air pump/and/or a filter pump is needed.