Josh testing for nitrites.  Nitrites can form from nitrifying bacteria that may develop in the tank.  The nitrites form from ammonia, from fish waste.  Both ammonia and nitrites can be poison to the fish.  Nitrates can then be converted to nitrates by other bacteria. Nitrates are not so bad.

Jeremy Whalen, Macie's dad, who works for Fish and Wildlife, teaches us how to siphon off waste to reduce ammonia levels.  It's tricky not to also siphon up the fish!