Ocean Alkalinity Enhancement (OAE) is gaining attention as a potential method for removing atmospheric carbon dioxide by enhancing the ocean’s natural carbon storage capacity. However, if too much alkalinity is added, it can cause unwanted chemical reactions, specifically, the formation of solid calcium carbonate. This reaction reduces total alkalinity and dissolved inorganic carbon, and can increase turbidity, ultimately limiting the effectiveness of OAE. Unfortunately, we don’t yet know exactly how much alkalinity is “too much,” especially across different ocean conditions, which makes it hard to accurately predict the outcomes of OAE in computer models used to track carbon removal. To help solve this, we ran a series of lab experiments to test when these unwanted reactions start. We added different amounts of alkalinity to seawater across a range of temperatures (6–30°C) and salinities (12–36), using both filtered and unfiltered seawater. We found that the high alkalinity additions led to more calcium carbonate formation in warmer waters, with filtered water being more sensitive to these reactions. Our results help identify safe limits for alkalinity additions under different ocean conditions. These findings will improve ocean models used to plan and monitor OAE projects, helping ensure these efforts are both effective and safe.