I have a 7" african black knife and it has a huge bulge in his stomach.I can't tell if it just ate a huge chunck of krill or if it has problems.Or could it be prego. If anyone has any idea let me know.

most likely it has had a huge snack, quick count your tetra population as soon as they get a taste for fresh food its the begginning of the end for your wee little tetra friends lol.

I tend to find the biggest problem with knives is that they loose weigth rapidly, but of course this guy has gained it. It could be the dreaded bloat or swim bladder problem, but i take it hes swimming fine at the moment.

As for pregnancy i doubt it, they arent that commonly bred in the home aquarium and it normally requires a few females and a large dominant male knife as well to get them in the mood