Untapping creatures is part of the cost of all of Halo Fountain's abilities. Once you've begun activating an ability, other players can't take actions until you have finished activating it, including paying its costs. Notably, players can't take actions to remove the creatures from the battlefield to prevent you from untapping them to pay for the ability.

Activating mana abilities is done before paying costs when activating an ability. This means that if you control a creature which can be tapped for mana, such as Avacyn's Pilgrim, you may both tap it for mana and untap it to pay costs for Halo Fountain's activated abilities.