Betta fry gasping!!

[pattyfurg]

This morning I went to go feed my betta fry (25) and I noticed the heater had broken. The fry were swimming slowly and staying near the bottom of the tank so I quickly unplugged it and replaced it with a new heater at the same temp (80 - 82F (didn't take the other heater out))
When I got back from school the fish seemed to be breathing really fast and not swimming at all. There was even a dead one so I tested the water and that were at:
Ammonia 0ppm
Nitrite .25ppm
Nitrate 0ppm
I freaked out at the nitrite so I did a 100% water change and they seem ok now. Except for 5 of them that still seem to be breathing fast and sticking around the bottom :S
There is some kind of brown algae in there but that has been in there since their parents were breeding in the tank and they are now afew months old...
Does anyone know what the problem is?

I also normally do a 30-40% water change every second day but iv missed about 3 since iv been busy.. Just listing all possible factors... they're really beautiful fish

 

[Atxpunx]

Could it have been that the heater breaking was shocking them? I lost an entire red cherry shrimp colony and a beautiful bristlenose pleco like that. If the glass on the heater was cracked, they may have been shocked.

 

[LyndaB]

You might want to add an airstone to increase their oxygenation.

What was the actual temp of the water when you put the new heater in?

 

[pattyfurg]

The second I saw the heater, yes it was broken in the morning so I unplugged it.
The heater was normally at 80 - 80F but the temp went down to the low 60's and when I replaced it I set the temp to 80 again. They're doing totally fine now and are showing no signs of harm