TLDR: nitrites are 5.0ppm and so we need tips on recycling our tank without harming our current fish
Okay so a few weeks ago we treated the tank with API General Cure for parasites. We took out our filter cartridge and had it sit in tank water forgetting that we needed to oxygenate the water so the beneficial bacteria didn’t die off. It sat in still water for an about 6-8 hours before we realized our mistake. We the added it into our 2.5 gal tank with cycled/oxygenated water but it was probably too late. After treating the tank, nothing odd happened and fish were okay so we figured maybe the bacteria was fine. We got a few new fish recently and no one showed any signs of distress or illness until this morning. One of our newer fish had a bloated belly (possibly dropsy) and didn’t make it. We tested our water and saw our nitrites were 5.0ppm. We did a 50% water change and added API Quick Start. We added our bubbler as well to increase the water oxygenation. We got 4 new neon tetras yesterday (already had 2 that have been thriving) and we know they are quite sensitive to any changes so we are surprised (and very relieved) that they are okay.
For more context, our tank is 29 gallons. All our fish are mollies, platies, and neon tetras. We have 2 bottom feeders. Lots of plants and tank light that’s on a timer for 8 hrs/day.
Please help! I don’t want our fish to die. What else do we need to do and monitor to make sure our fish are okay? I heard adding seachem prime can help protect them, thoughts? What about aquarium salt?