Why are my sword-tail fish dying?

First things first. What size tank is it? And do you mean plecostomus? Sounds like a lack of cycling and possibly overcrowding issue. For the fish you have (10 sword tails) you would need at least a 30 gallon tank not counting the pleco which can grow to nearly 2 feet, the frogs and the guppy. Even if the tank was cycled, adding 5 sword tails at one time is a recipe for disaster. It overloads the beneficial bacteria in your tank by adding too much ammonia at one time. Never add more than 2-3 fish per week no matter what size tank you have. PLEASE post your tank and filter information so we can help steer you in the right direction to keep some of these guys alive.