"It out-competes our native fish" says Dave Mee, from the Environment Agency.
"Water becomes totally dominated by these small fish."
"They're small, they're a nuisance and they're not viable for anglers to fish for, so they're not a targeted species".
A major project is underway in Llanelli to clear the lakes in the coastal park of an invasive fish.
Topmouth gudgeon is a small freshwater fish from Asia. It was brought to Europe accidentally in the 1960s.
Experts say it's a highly damaging species. It carries a parasite that attacks other varieties of fish, and dominates the habitat and food supply.