Tank Catastrophe, But Why?

[ParadiseGuerrero]

I woke up this morning to find five dead rosy barbs and all the other fish acting funny in both my tanks. Test the water to find 0 ammonia but in the small tank nitrites were off the charts and there were a good few in the other tank. I did a huge water change and they all seem fine now but I want to know what might have caused this.
I thought at first perhaps we had a power cut or something but the clocks were still running so it can't have been. other thank this I don't know what could have caused the huge spike. They've both been cycled for quite some time and I've not added anything to them within the last week.

Any ideas? I really don't want this happening again.

EDIT:I'd just like to add I have an awsomly smart shrimp- the water went bad so it climbed out onto the ledge until I had finished the water change then jumped back in- it's lucky amano shrimp can walk out of water

 

[ParadiseGuerrero]

anyone? or is this just a fluke?

 

[fishprotector]

Sounds like a mini cycle. If you have not added anything for a while and the power has not been off, then I would have to ask have you overfed them or had anything die in there?

 

[ParadiseGuerrero]

I did not feed them that night- it was a starve day and it happened in both tanks. Nothing died in the big tank but during the night 5 rosy barbs died in the small tank, I'm assuming that's why it had atrociously high nitrites in there compared to the other tank but I can't think what would cause them all to die in the first place-I gather the levels shot up for some reason but it just seems odd that I should have a mini cycle in both tank at the same time?

 

[afremont]

I did not feed them that night- it was a starve day and it happened in both tanks. Nothing died in the big tank but during the night 5 rosy barbs died in the small tank, I'm assuming that's why it had atrociously high nitrites in there compared to the other tank but I can't think what would cause them all to die in the first place-I gather the levels shot up for some reason but it just seems odd that I should have a mini cycle in both tank at the same time?

Maybe overfeeding caused a spike in the ammonia and nitrItes. The ammonia spike would be short lived, but still could have been high. Any possibility anyone is "helping" to feed the fish when you're not looking?

 

[ParadiseGuerrero]

I know that no body would touch my tanks..they value their lives I'll cut back on the feeding just be sure.