two swordtails will be maximum capacity for that tank. in fact, I wouldn't even do that...I'd put one betta in there, maybe a ghost shrimp and call it done.
You have to test the ammonia and nitrite. If ammonia goes over .5ppm and you have fish in there, do a 50% change. if nitrate is over 1ppm, do a water change.
once the tank is cycled, a weekly 50% change is all you need.