Recommended Tank Size For One Swordtail

[aaaflaaaak]

So I have a swordtail in a 3 gallon tank who has been doing the shimmying motion a lot. I changed the water, checked acidity, increased temperature, added aq. salt, etc all are fine but he's still shimmying. Could this be a parasite or just a tank that's too small?
 
He had been perfectly fine a couple months ago when the weather was even colder (and before i got a heater). He's also not eating as much. HELP!

 

[TallTree01]

Minimum tank size for any fish is 5 gallons ( betta or guppies ). Minimum for sword tail is 20 gallons due to there 4 inch size potential and activity level.