Really Weird Nitrite Problem! 0.0 To 1.0 In 2Hrs!

[jnms]

I have two filters a Fluval 305 and a Fluval 405 running on my 200ltr tank. Both filters are fully cycled and have been running for 3yrs with regular cleaning adn maintaince.

After doing a water test, I found the Nitrites were over 5.0ppm!!

So I did a 80% water change, tested again and the Nitrite was 1.0ppm. I did another 80% water change which brought the nitrites down to 0.0ppm. That was 2hrs ago. I just did another water test and the nitrites are back to 1.0ppm again!

All I have in the tank is 17 mollies and 1 bristlenose pleco. There is no organic matter in the tank, and no dead fish or plants.

The only cause I can think of, is that the Fluval 305 filter was left switched off for 36hrs. After it came back on, could it now be spewing Nitrite and Ammonia into the tank?
Could there be anything else causing the nitrites to skyrocket like this in just a few hours?

Edit: The Ammonia is now at 0.125 - it was at 0.0 2hrs ago.

 

[This Old Spouse]

I would think if the filter was off for 36 hours, the media would start to rot, hence the elevated ammonia and nitrite. Get the media out of that one and take some from the 405 to get it cycled again.

 

[jnms]

Thanks. I will take out the wool and the sponges.

Will it be ok to leave the ceramic parts?

 

[This Old Spouse]

Good question. I would think if you rinsed them out really well in old tank water that should do. Same for the sponges (squeeze, squeeze).

I'm not an expert on these filters at all, but I'd say just get it clean as possible.

 

[Smells a bit fishy]

I don't think that's necessary as 36hours you won't have lost all the bacteria in the 305. Its just a mini cycle and moving medias around won't achieve much. If I were you I would just test daily and do water as and when. It will sort itself out in a few days

On a matured tank and filters. The bacteria will bounce back before you know it

 

[jnms]

I wondered if it might be a mini-cycle. But I just don't see that 17 mollies could create 2.0ppm of nitite in a 200ltr tank inside of 3hrs.

In addition the 405 filter is perfectly fine - whilst that was running on its own, there was no nitrite build up at all. As soon as the 305 switched back on, the nitrite levels started rising.