Tropical Fish Keeping - Aquarium fish care and resources

Tropical Fish Keeping - Aquarium fish care and resources (http://www.tropicalfishkeeping.com/)

- Tropical Fish Diseases (http://www.tropicalfishkeeping.com/tropical-fish-diseases/)

- - Dropsy? (http://www.tropicalfishkeeping.com/tropical-fish-diseases/dropsy-122340/)

My 100 l tank has 15 tetras, 5 cory's and a betta. All water parameters are fine, temp 25

2 weeks ago 2 of the tetras started to puff up, I thought they were pregnant but now my beloved betta Seraphin has started to pine cone. The tetras seem to be breathing very rapidly and Seraphin is completely lacking in energy. I fear I'm about to loose all 3. Does this sound like dropsy? I thought it was non contagious?

If its dropsy I know there's not much I can do as its usually fatal :-(

2 cory's were added 3 weeks ago but appear perfectly healthy?

One of the tetra's is now gasping at the surface, I've checked my water again, ammonia 0, nitrite 0, nitrates 20, I have turned the lights off and dropped the blind in the room as I don't think Seraphin has much longer left. PLEASE, does anyone have any idea what this is?

One of the Tetras has died, 2 more look bloated and my betta is just floating around. I just don't know what to do