The blue-scaled fish was attracted to the manifestation of Eren's water-element spells. It started swimming toward them without any fear for its well-being.
The fish aligned its horn to face the blades of water approaching it at a critical speed. In the next moment, however, the blades crumbled and an abstract concept was separated from them. The blue-scaled fish's horn shone in bright blue as it opened its mouth and absorbed the concept.
The same thing happened with Eren's Aqua Whips. They lost their shape and turned into normal water when they approached the fish. The water returned to the lake while Eren's elemental essence was absorbed by the fish.
Eren and Kirin both looked at each other when the phenomenon took place in front of their eyes. Since the fish had the power to shake the very building block of the spell, they weren't sure what they could do to catch it.
"Use your water-element spells, Eren."