For both the three and six months ended March 31, 2021, mutual fund distribution expense as a percentage of total revenue increased 0.2 percentage points to 1.5%. The increase in mutual fund distribution expense as a percentage of total revenue was due to a larger portion of the fees being attributable to fixed minimum fees, which do not fluctuate in line with revenue.
Sub-Advisory Fees Expense: Comparing the three months ended March 31, 2020, to the three months ended March 31, 2021, sub-advisory fees expense decreased by 10.2%, from $2.0 million to $1.8 million. As a percentage of total revenue, sub-advisory fees expense increased 0.1 percentage points to 22.7%.
Comparing the six months ended March 31, 2020, to the six months ended March 31, 2021, sub-advisory fees expense decreased by 17.0%, from $4.3 million to $3.6 million. As a percentage of total revenue, sub-advisory fees expense increased 0.2 percentage points to 22.8%.
In both periods, the dollar value decrease in sub-advisory fees expense was due to decreased average daily net assets held in the sub-advised Hennessy Funds. Although the dollar value decreased, sub-advisory fees expense increased as a percentage of total revenue because the decrease in average daily net assets held by the Hennessy Funds that we internally manage was larger than the decrease in average daily net assets of the sub-advised Hennessy Funds.
Depreciation Expense: Comparing the three months ended March 31, 2020, to the three months ended March 31, 2021, depreciation expense increased by 3.3%, from $0.061 million to $0.063 million. As a percentage of total revenue, depreciation expense increased 0.1 percentage points to 0.8%.
Comparing the six months ended March 31, 2020, to the six months ended March 31, 2021, depreciation expense increased by 9.6%, from $0.11 million to $0.13 million. As a percentage of total revenue, depreciation expense increased 0.3 percentage points to 0.9%.
In both periods, the increase in depreciation expense resulted from fixed asset purchases.
Interest Expense
Comparing the three and six months ended March 31, 2020, to the three and six months ended March��31, 2021, interest expense decreased by 100.0% from $0.26 million and $0.45 million to $0.
In both periods, the decrease in interest expense was due to the payoff in full of the remaining outstanding balance under our term loan agreement with U.S. Bank National Association on March 26, 2020.
Income Tax Expense
Comparing the three months ended March 31, 2020, to the three months ended March 31, 2021, income tax expense decreased by 14.8%, from $0.8 million to $0.7 million. Comparing the six months ended March 31, 2020, to the six months ended March 31, 2021, income tax expense decreased by 25.6%, from $1.8 million to $1.3 million.
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