One of my tadpoles has become very bloated looking. He has his back legs. Do tadpoles get fatter before they change or is he sick? It seems to have happened suddenly and I hope he isn't dying although it doesn't look right to me.

May have a disease but i think it is just metamorphosing. What type of frog is it?

It's a bullfrog. It seems to be acting normally i guess... its just so fat that it looks like a ball and it wasnt like that a few days ago.

The tadpole may have an internal bacterial infection or bloating/gases. Once tadpoles start to grow legs and change, they normally lose body mass as it takes lots of energy to morph. I'm not sure what treatment might help. Some salt (a tablespoon per 3 gallons or so) might help. Antibiotics probably won't help much unless injected.

Sadly, the little guy was dead at the bottom this morning. It must have been a bacterial infection because if he was bloated with gasses wouldnt he have floated? Do u think that bacteria is contagious? there are a couple other tadpoles in there.

Being bloated doesn't necessarily mean they would float but often they do. Bacteria usually take a little while to grow while bloat can come on quickly. If bacteria were involved, the other tadpoles were exposed but hopefully they are healthy and will easily fight off the bacteria.