Sorry to hear your trouble Stephanie.
When you vac the gravel do you see tiny 1/4inch thread like worms floating and wriggling about. The reason I ask is I had my goldfish do this a few months back and then it died. I then lost my bristlenose catfish and white clouds. I noticed that after the goldfish died and I did a gravel vac/water change these little thread like things had been stirred up from the gravel. I didn't know that they were going to harm my other fish but discovered the rest of them dead the next day.
Pls. check and if you do see them try to put your other fish in a diff tank. Treat you tank with a worm rid medication. I am not sure which as I started my tank over from scratch 1 month after the problem.
I hope all goes ok, just be careful when you do a gravel vac
I am just letting you know my experience with similar symptoms as your fish. I am only new to this hobby and maybe other more experienced members can advise better.