So my Narrow Lined Puffer had been perfectly fine up until this morning when he had been floating upside down for extended periods of time and his back had gotten a darker blackish color to it, it seems like he's struggling to swim down and not float to the surface and tries to use rocks to hold himself down, when i fed him he did eat although he tried to eat upside down. I had him in a 10 Gal tank up until this and when i tested the water the nitrites were 2ppm, so i started to acclimate him into a new 29 gal tank but the problem persists, hes been in there for at least 45 minutes, while in the bucket he was able to sit right side up however. I tested the water of the 29 gal and it was ~0.25ppm. Both tanks had low ammonia (~0.25ppm) and normal nitrate (>20ppm). I'm relatively new to the hobby and I'm not sure if he's having a problem with the water or some kind of sickness and cant find many things or posts on forums talking about this kind of issue.