Something Wrong With All The Fish In My Tank!

[James Bond the fish]

Here's all I've done in the past few days:

Friday: Installed an airpump + airstone, bought two small angels
Yesterday: Found my betta with a severe case of finrot which I detailed about here.
This morning: Found siamese fighting fish dead in the livebearing tank.

Now I had a feeling that he wouldn't make it, he looked very sick. However, there's something wrong with the rest of the tank too! All the fish appear "stunned", staying very still and not responding at all. I put a few flakes of food in and they just sank to the bottom, one of my swordtails took one of them but that was all (they're all normally very hungry and competitive for food!). I even brushed my finger against one of the angels and it just floated away, and for a second I thought it was dead. My female betta seems the worst affected, actually hiding under a plant and looking like she might die at any moment.
Action I've taken: Done a 50% water change, cleaned out the filter sponge in the tank water.
So far this seems to have done some good, with every bucket of clean water I put back in, the fish seemed to animate more and more. I've done a test for all the usual and got:
Ammonia: 0
Nitrite: 0
Nitrate: 12-25
pH: Not sure, my LFS sold me a marine test kit which I need to exchange soon, but it's usually around 7.0.
My tank consists of tetras, livebearers, the two angels, corys and kuhili loaches.

 

[stang1]

What size tank is it?

 

[DarkEntity]

DId you cycle the tank?

How large is the tank?

 

[bubblesburst80]

hope your fish recover

xXx

 

[LuckyCricketMafia]

Could something have gotten in the tank?

 

[James Bond the fish]

The tank is 3' x 1' x 1'. It's been going since about April/May, so is quite well cycled and doing well with few deaths until recently. The tank is covered up with only small holes to lift the lid up and let leads through.

EDIT: Thanks bubblesburst, most of them seem "okay", the female betta doesn't look so good though.

 

[afishdude]

wait you had a female betta, a male betta, and 2 angels in the same tank?

 

[James Bond the fish]

Yes? Never had any issues with any of them in the short time they were together.

 

[afishdude]

that "fin rot" you saw was prob the angels atacking the bettas and them fighting at night or when you wernt paying attention. bettas can not be kept with angels. and you cant put males with females because they will stress each other out. thus weaking the fish and leading to fin rot and other dieses.

 

[James Bond the fish]

I've honestly never heard of that... else I wouldn't have put them together... I always thought bettas prefered slow-moving fish over faster ones, and figured the angels would be good tanks mates. Never saw the angels as problem-fish during the early stages.
Still, doesn't explain why all the fish act the way they are doing.

 

[afishdude]

no it doesnt but it would explain the male betta death.

 

[James Bond the fish]

Well last night the female betta made her home in the filter vent which is where I found her dead this morning.
One female guppy was found dead inside one of the planets.
One neon tetra was also dead in the plant.
None of these fish showed any sign of illness until that one day I found the betta torn and added the finrot/fungus chemical to the water.
The angel fish stay at the back near the pump, while the other fish are at least swim about a little but none of them are acting like themselves (guppies aren't chasing the females, black widows are "lurking" instead of shooting around the tank)... it feels like the entire tank is falling apart while the fish show no signs at all that I can distinguish and cure!

 

[afishdude]

can you re-test the water stats?
and do a 50% change now.