Although strange that it has happened twice, Roundup is unlikely a contributing factor - assuming you weren't treating aquatic vegetation.
Did any unusual weather events coincide with those two treatments?
Some details about the fish-kills would be helpful. Specie(s)? Big fish? Small fish? Both? Did they seem oxygen-stressed or lethargic before dieing? Are you aerating the pond? etc


Kelly Duffie
Cypress, TX