While the Aquatian Rangers were on Earth, they fought the Earth Ranger's enemies. However, they had one main enemy from back home on Aquitar, who at one point did pay them a visit on earth.
Hydro Hog is the Alien Rangers' arch enemy. Lord Zedd summoned him to Earth to help him defeat the Alien Rangers by draining the Earth's water supply with his power of evaporation. Hydro Hog was destroyed by the Alien Rangers.