Any nitrite levels in an established aquarium are a sign of something very wrong.
, you most likely have a combination of too many large fish with too little bio-filtration. I would look to return some of the fish or get a larger tank along with a bulkier filtration system.
The nitrites must be taken care of immediately by reducing the bioload as mentioned and/or getting a more efficient filter that you can move onto the larger tank. A large wet/dry system would be more efficient than HOB
filters. Large water changes could help you in the meantime reduce the nitrite levels.