Why did it crash? What's the cause of alkaline...YOUR ANSWER IS UNCLEAR
Simply put, Alkalinity is a measure of the acid-neutralizing capacity of water. The lower the Alkalinity the less buffering effect it will have on pH changes. In earthen-bottom ponds an Alkalinity level of at least 30 mg/L is recommended. In Garden Ponds, much higher levels of Alkalinity are often recommended.
Bacterial action, either in a biofilter or in the degrading process of other organic matter, generates acid, As this acid is produced it will be neutralized (Buffered) by existing Alkalinity. This gradually reduces the Alkalinity level in a pond and when the Alkalinity drops low enough (usually <20 mg/L) a pH crash may occur. In Garden Ponds where naturally occurring Alkalinity is not available, the simple addition of Baking Soda can be a temporary solution.
Alkalinity should be tested before any attempt is made to adjust the level.
