Keaf, murk of the ghoti

Whenever you control fish creature, you may cash this spell rather than pay this mana cost

Whenever creatures enter to the opponent battlefield, you may target that creature and 1 your exiled fish creature with mana value 6 or less: exile this creature and opponent target creature, then put it on the battlefield your exiled fish creature

During the upkeep of the next turn, whenever this card was exiled: put it on the battlefield