The bride- and groom-to-be mark a big milestone, getting a fish together and naming him King Henry V

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Amy Roloff and fiancé Chris Marek are marking a big milestone in their relationship: getting their first pet together.

In PEOPLE's exclusive sneak peek at Tuesday's episode of Little People, Big World, the pair bring home a new fish to replace Marek's old fish Henry, who died.

"This feels so important," Roloff says.

Together, Marek, 59, and Roloff, 56, place the fish in his tank, watching him resist for a moment before he settles into his new home.

"I dub him King Henry V," Marek says, adding, "just King Henry to his friends."

Roloff also has dog Felix, who comes over to check out his new competition, as Marek reminds him, "You're still number one in this house."

Marek and Roloff are planning to tie the knot on Aug. 28 at Roloff Farms, the property where she and her ex Matt raised their four children, twins Jeremy and Zach, 31, daughter Molly, 27, and son Jacob, 24.

Roloff calls the experience "exciting," while Marek says it's one of the many "little baby steps we've taken that couples or life partners do together."

Says Roloff, "I think it's a big move for him."

Little People, Big World airs Tuesdays (9 p.m. ET) on TLC.