Okay it could be a mini cycle in the 5 gallon,
Or the bioload could be too much for that filter, I know pea puffers don't get big but it's recommended at least a 10 gallon aquarium I would get a bigger filter or *** a small sponge filter to help, I'm thinking a mini cycle.
I was thinking the mini cycle Too for the 5 gallon, it’s the only thing that seems to make sense to me, I’m going to put the puffer in a breeders net in my 29 gallon tank for the meantime and if it happens to pass I’m debating just taking the 5 gallon down