can you let us know what the water stats are in numbers, not just 'fine' please.
we need pH, ammonia, nitrite and nitrate.
can you also give us the make and model of the filter and describe your maintenance routine to us.
what is the name of the medication that you're using, don't bother with the general tonic, they're no good.
first sign of trouble you should always do a large water change to make sure the water is clean and healthy, so once you've taken the readings you should do a 50% water change straight away.
you shouldn't keep male guppies with a male fighter, don't think this is the cause of your problem but it's probably not helping, the guppies long flowing tails look too much like another fighter and it stresses the fighter out, they either just get stressed or attack the guppies and they are more than capable of killing them. you also shouldn't really have tetra's with fighters, they nip at the fighters fins again stressing them out.
there's so many fish that you can't keep fighters with that it's generally recommended that they are kept in their own tanks of around 5 gallons so at some point in the future you may want to think about a little tank for him. He'd be much happier in there.
