"Decrease in Interest" should manually add Interest amount in "Gross Interest" section as tax facts to make the difference amount to zero amount. If there are differences between accounting and tax facts, the differences will be reflected in the Reconciliation section of the form. Increase/decrease in Interest should be nil.
This also applies to income received from Dividends, Trusts and Partnerships.
You need to input the same amount of interest income to the "Gross Interest" line item as tax fact (example amount in this illustration is $494). After inputting the tax fact detail in "Gross Interest" line item, the "Increase/Decrease in Interest" line item will be zero "0". See below image for your reference:
Guides:
Reporting Interest Income received into a Company Tax Return (CTR)