Reef-building deep-sea corals are facing new threats from climate change, including changes in ocean chemistry. These corals are especially vulnerable to increasing ocean acidity (i.e., ocean acidification). In the ocean, the aragonite saturation horizon (ASH) marks the depth below which waters become increasingly corrosive and deep-sea corals’ aragonite skeletons may dissolve. As ocean acidification progresses, the ASH is moving shallower, which can endanger the health and survival of these deep-sea reefs. Our study examines water chemistry near deep-sea coral reefs on seamounts in the Hawaiian-Emperor Seamount Chain in the North Pacific from 2014 to 2019. We observed spatial changes in water chemistry, particularly at depths where deep-sea reefs have been found, that include shallower ASH and lower oxygen levels in more northern waters, which could limit where reefs can grow. Additionally, seasonal changes may also shift the ASH depth by over 100 m, affecting local reef conditions. Understanding these chemical conditions and how they can change is crucial for managing and protecting these important deep-sea coral reef ecosystems as climate change and ocean acidification continue.