Funnily enough lyretail swordtail wasn't one of the three species I'd thought of.
What type of fish is it? Lyretail swordtail
How long has the tank been set up? 8 months
How did you cycle the filter (ie grow the bacteria colonies)?
What are your water stats for pH, ammonia, nitrite, nitrate?
How big is the tank (volume and dimensions)? 260l
What else is in the tank? 2 other swordtails
Have you added anything else to the tank recently (living or otherwise) plants and bogwood
How often do you do water changes, and how much do you change? Weekly 30%
Have you put any chemicals in the water recently? None
Hope you don't mind that I've summarised like that.
"Fine" as a set of water stats doesn't really help I'm afraid, I assume it means 0ppm for ammonia and nitrite, but doesn't tell me your level for nitrate, and it doesn't tell me what your pH is. Also you didn't tell me how you'd cycled.
When a fish reacts like that and there are no visible signs as to why (I assume you'd have mentioned anything obvious), it's often down to water quality problems - whether historical or immediate.
Also, how come there's only 3 fish in a tank like that?