Sudden death of juvenile Koran Angelfish and Sea Betta

[rockeyemma19]

I found my Koran Angelfish dead on the bottom of my tank last night. She had been swimming around 3 hours before, and I had hand fed her earlier that day. Nothing seemed unusual or out of place leading up to her death, it appeared to be completely random. Tonight, I came home from work to find my sea betta floating at the surface on his side with a clear bubble coming out of the bottom of his abdomen. I also noticed that one of my mollies had disappeared, and I assume the sea better ate it. I have a 24 gallon nano cube with only the sea betta, angelfish, and 2 mollies that were left over from cycling the tank. I was keeping the angel and betta for my friend temporarily as he sets up his 75 gallon, and had housed them for 2-3 weeks before they died. They were perfectly healthy leading up to the death. The tank is well established, but had experienced a total wipe out in September when the power went out when I wasn't home to fix it. The angel and betta were the first fish introduced after the mollies, who had been in the tank by themselves previously for over a month.
Temperature: 76 degrees
Salinity: 1.2-1.3 (haven't checked in a month, have done 1 water change since the wipe out)
pH: 8.0
Ammonia: 0
Nitrite: 0
Nitrate: .8

I have no idea what could have happened. I wonder if the sea betta died from impaction after eating the molly, but that doesn't seem likely. I have never seen a bubble come out of a fish that way. It appeared as though it was coming out of his abdomen. Another possibility could have been starvation, because I was unaware of their feeding routine and had been giving them Angelfish Formula One frozen cubes and Omega One flakes on alternating days of the week. I have no idea what happened to the Koran Angelfish. My heart is broken over this loss, especially following the wipe out.

 

[Sniperhank]

This is most likely from aggression if not parameter issues. These are big fish that need big territories and will beat on one another when that is disturbed.
The 'sea betta' you are talking about the comet, aka marine beta? Just want to ensure we are talking about the same fish. I have one and adore mine. How big was it? It will eat small fish, but to eat a molly it would have to be a very large beta.
What test kit are you using for testing? I personally have trouble seeing that nitrate reading with so many fish crammed into such a small tank. One large fish has the ability to greatly increase nitrate levels in a system, big fish = big poop sort of deal. The same goes for the establishment of the tank, die off or not, the introduction could have easily overwhelmed the current bacteria base in the tank when they were added and caused a mini cycle. Could explain all of the deaths.
With all that said, the main thing about this hobby is to take thing slow. The faster you move the more likely tank crashes and mistakes are to happen.

 

[rockeyemma19]

I don't think they were aggressive towards each other. The friend who gave them to me had a slightly larger tank that he wanted to break down and they lived together in it peacefully for 6 months before moving to my tank. The sea (marine) betta was approximately 4 inches and the angel was slightly shorter in length. They were getting along well in my tank also and I never saw either of them pick at or chase each other. The betta mostly hid in the caves made by my live rock. Your point about my large bioload makes sense because the tank definitely did not have the proper amount of bacteria to support both at the same time. The deaths seemed so sudden though, I thought I'd notice if they started getting sick or acting strangely. They both looked perfectly healthy up until the moment of death which is what is so disturbing for me... I still don't understand the bubble on the bettas stomach or the Molly disappearance if it wasn't eaten. I am using the API master test kit. Nitrates were bright red when I tested on Saturday night.

 

[Andy Sager]

Gas bubbles are usually a sign of too many tiny bubbles in the water. The Koran may have been a different issue. The timing suggests that it ingested something when you fed it by hand. It could have been a contaminate on your hand or something bad within the food.
Does any of this sound like your situation?

 

[Sniperhank]

Smells like a mini cycle and very high nitrates left over from such to me.