Nitrite spike

[terrypin]

Hi I have 150L tank and the filter set up is allponds 1000 cannister which carries 3 baskets the bottom one is both coarse and fine sponge only the the other two carry bio home ultra with a carbon/zeolite bag to finish. This filter I changed over to bio home recently one basket at a time a month apart we are 2 weeks into the second basket. I also have an all ponds pre filter which has 3 sponges coarse medium and fine these I thoroughly clean weekly in boiled warm water with a final rinse in a bucket of tank water to protect the bacteria in the main filter. Last night about an hour after cleaning the pre filter one of my rummy nose was upside down gasping on the surface also more than normal amount of shrimp in the floating crystalwort. I noticed that I had turned down the air stones by accident so turned these back up and tested the ammonia and the nitrite. Ammonia between 0 and 0.25 but Nitrite was 1.0ppm. Did a 30% water change and tested filter water at outlet slight drop 0.50ppm fish no longer in distress. This morning have done a further 30% water change no losses as far as I am aware. tested water from filter outlet is now ammonia 0.25 and Nitrite 0.25 also tested water from the bottom of the tank but reads the same fish behaving as normal including shrimp. water changes treated with seachem prime and stability also liquid co2 and teaspoon of salt every time I do 30%. What do you think caused the spike did I upset the filter bacteria in some way. I have to go into work soon but will do another change tonight and again tomorrow morning testing both.
Also I have two nets in the tank full of platy fry and molly fry no losses in there

 

[PheonixKingZ]

How are the readings now?

 

[terrypin]

readings back down to zero still not sure what happened tested again last night Nitrite zero no water change then again this morning still zero cant understand why the nitrite was so high and then recovered so quickly. will do water change again today and testing again later. Don't appear to have any losses even the fry are lively and eating well. I was concerned that changing the media the bacteria was still not established well enough and the cleaning of the pre filter had an adverse affect on it. Even the shrimp gone back into hiding only the odd one sitting in the crystal-wort as normal