Swordtail fish is twitching?

We have a swordtail fish in a community tank. Three angelfish, one tetra, two catfish, dragon goby, two sharks, jack knife and then the velvet swordtail. The newest fish is the dragon goby but he's been in the tank for about a month and this problem just started with the swordtail about 4 days ago. He's swimming kind of sideways and at first I thought it was a swim bladder problem but he's twitching every couple of minutes like he's having a seizure. He just started hiding behind plants, etc. today so I am sure he will be gone tomorrow. But I'd like to know what this is and if its going to effect my other fish.