No one mentioned this yet... CO2. I use this method to kill all the scuds.
Overdose your tank with co2 gas for 24 hour. Filter, heater and lights off. Lay a layer or two Saran Wrap on top of the water surface (maximizing co2 concentration in the water). Basically it.
This worked for me so far when I'm trying to transfer all my plants from an infested tank. This is also what I do to quarantine new plants from anywhere I bought. The best thing about this method is chemical free so plants won't be affected.