What killed my fish?(And is the hope for my other?)

[Korbexo]

I'm pretty upset right now so I'm sorry is this get's long and/or confusing but I'll try to have it make sense. I'm at a loss as to what happened and I'm extremely worried for my one surviving fish, so any help is very much appreciated.

Last night I went to bed around 3:30am, and before doing so checked on my boys in their divided 5.5 gal tank. The water was clear, their color was vibrant, and they were both very active, flaring at each other and even me. I actually had friends over and was telling them about how much I love my fish and how upset I would be if I lost them...
Well lo and behold I wake up at 1:30pm today, check on them, and the water is cloudy, my boy Chaos was lying on the bottom barely breathing(I thought he was dead) and my other Nen had wedged himself between the thermometer and divider towards the surface, huge silver stress marks across his face and body. I was shocked!

In my hurry to get the bad water out of there I didn't even think to check the water parameters, but when I had the day before it was under .25ppm ammonia, 0 nitrites, and about 10ppm nitrates. The water has been at a steady 80 degrees all week.
As soon as I noticed their condition I changed as much of the water as possible without having to move them, as I did my boy Nen swam around, but Chaos wouldn't move. Only 5 hours later and Chaos is now dead...

I have a theory, but I don't know how much it makes sense,
I had people over and was having a party, so the light in their tank was on a lot longer than it normally would be(five or six hours more), in the tank was two live plants, a broad leaf anubias and a sword anubias. When I was changing the water I noticed that two of the broad leafs leaves were brown and curling in on each other, and almost all of the sword leafs leaves were brown, limp, and slimy. A lot of it's roots were falling off as well. My thought is that the light being on so much killed the plants, and as they decayed the ammonia spiked and crashed the cycle and good bacteria.
If that is the case, what should I be doing for my remaining betta? The stress stripes on his body are gone, but his face is still stripped and silver, and he is once again wedged between the thermometer and the divider. I tried to give him food, and while he swam up to it with interest, he quickly swam away again without eating.
I removed the sword anubias, and trimmed the dead leaves off of the broad leaf. The rest of the plant seems in good condition but I'm still wary. Should I be moving Nen out of the tank? Current parameters are under .25ppm ammonia, 0 nitries, and 0 nitrates. It is filtered, and had been cycled but now I'm not too sure of that...
On the other hand, could this have been the effect of some disease? It happened so fast, within 11 hours, it seems unbelievable but I just don't know.
I'm extremely worried about my Nen, he is still very inactive and I'm afraid I'm going to lose him too. I just want to know what went wrong so I know if it was a fault of mine or not. Chaos has been with me for almost a year and a half and I'm devastated he's gone. :(

[callistra]

Having few hours extra light one day won't hurt your plants... may even create more growth. However it did seem like they weren't healthy for whatever reason and their decay will increase the amount of ammonia. If your biofilter isn't strong enough to handle it you could see an ammonia spike.

You're saying under .25ppm ammonia and not zero.. so if you are still seeing some ammonia after such a large change you must have had a lot of ammonia to begin with.

How long has this tank been set up? How often and how much do you normally do water changes?

Also.. hate to say this.. but you sure none of your friends dumped something in the tank?

Does your remaining fish look and act normal now?

[Korbexo]

The tank itself had only been set up for 6 days, but the filter and all of the gravel and decorations had come from my cycled 10 gallon at home.(I just moved back to school this past week.) I changed the water 50% about every three days, and check parameters every morning. The amonia has never gone to or above .25, but it hasn't been 0 as of yet in the 5.5 gallon. I figured it was because of the move and added burden of two fish in a smaller tank.(The filter had previously been in a divided 10 gallon.)
The same thought did cross my mind, but I had been sitting next to the tank all night, and am 99.9% positive that nothing got into the tank. The lid stayed on and the feeding hatch closed.
As for my remaining fish, the stress stripes on his body are gone, but they're still on his face. He's also gone back to resting himself between the thermometer and the divider and isn't swimming around like he normally would. He also refused food. His color besides his face looks good and vibrant, and his fins are all in tact, but his breathing is shallow. He can swim around though(Chaos couldn't), and when he does he isn't struggling, but he looks kind of lethargic.

[callistra]

Can you share photos?

I think it's good you did a large water change. You may want to do another 50% tomorrow.

[LittleBlueFishlets]

Do you have a hospital tank? If so, you may want to set it up with new water and move him into it. That would give you some time to figure out if the water in the big tank is OK or not.

I see it like this: if a person is in a room where the air is bad, the first thing you do is take him/her outside into fresh air.... So if a fish is in bad water, I'd want to move it into clean water, in case something 'bad' is still in there. And after he's in clean water (and safe), you can try to figure out what happened to the tank, without worrying if something 'bad' is still in there and affecting him....

[Korbexo]

Unfortunately I only have my phone at school with me and it takes terrible pictures, but in the one I did get you can see the striping in his face in comparison to how he was healthy. It's been hours and he's still in the same spot. He did move and swim away a bit when I shined the light in his direction, but then he went right back over.
I do have a 1.75 gal hospital tank, so I'll set that up and get him in there tonight.
If I set him up in there, do you recommend that I just keep up water changes on the 5.5 gal, maybe 50% a day, and if so how many before I put him back in?(If he makes it...I really hope he will.) Also should I do 100% water changes on the hospital tank daily, or will that be too often and stress him out?
Here's the comparison picture, in the one on the left you can see that his face when healthy was very dark. I apologize for the terrible second photo, but you can see the silver, almost white streaking and "beard." :(