Fish dying help!

[Marvensmom6]

I have a 26 g tank and I bought a new cannister filter and forgot to add old media to seed it. My tank has crashed and there are no nitrates. It gets worse though. I had kept old media sponge in a small cup but it smelled really bad but I put it in new filter anyways as my tank
Had crashed.

Woke up this morning and 4 fish were dead.Did
50% water change and added prime and some biological enhancer. When I consulted with local fish store she said to start filter again even after the old media had started to grow in filter media.

I’m worried that filter is spewing bad bacteria into the tank. I took the old media out.

Today Nitrates did not show up in test and I
have tiny amounts of ammonia and nitrites.

What should I do now? Should I clean the filter media to rid it of bad bacteria or…?
Please advise, thanks

 

[Marvensmom6]

Forgot to mention that one fish has a white spot on him. Please help!!!

 

[Aiken Drum]

You crashed your cycle. The media no longer has enough microbes that consume ammonia. The ammonia is killing the fish. You say that there is a tiny amount of ammonia, but this is after you did a big water change so the ammonia will have diluted down and be lower than the level it was when the fish where first exposed to it.

The sponge isnt likely to be the cause, but take it out anyway as a precaution.

The guy in the store is correct, you need to cycle the tank again. Do you know how to do this?

Edit: Yes you do.

https://www.aquariumadvice.com/forums/f15/ammonia-and-cycling-tank-sos-379434.html