Bitcoin Spot ETFs Add $433M on Sept. 18, Yet Holdings Drop 5,700 BTC Over Two Weeks

U.S. spot Bitcoin ETFs logged $433 million in net inflows on Sept. 18 (ET), according to SosoValue, led by Fidelity's FBTC with $311 million. BlackRock's IBIT followed with $108 million. Even with the one-day intake, Bitcoin ETF holdings declined by about 5,700 BTC over the past two weeks, marking a second consecutive weekly drop. Analyst Darkfost attributed the broader weakness to risk-off headlines, including rate-hike concerns and geopolitical conflicts, noting that the Sept. 18 surge was not enough to flip the weekly trend. Spot Ethereum ETFs also saw fresh demand on Sept. 18, posting $144 million in net inflows. BlackRock's ETHA accounted for $114 million, while Fidelity's FETH added $26.2449 million. Market context from January 2024 through September 2026 shows ETF flows oscillating between large inflow spikes and periods of meaningful redemptions. In early 2024, daily swings were choppy, with several inflow bursts approaching $1 billion. Flow momentum strengthened from October 2024 into early 2025, when multiple spikes topped $1 billion and the largest reached roughly $1.3 billion to $1.4 billion. From mid-2025 through late 2025, Bitcoin's price action improved as ETF inflows frequently ranged between $500 million and $1 billion, before outflows picked up again, including withdrawals in the $500 million to $1 billion range. In 2026, the pattern remained mixed, with intermittent inflows above $500 million repeatedly interrupted by outflow sessions. Darkfost said Bitcoin still posted a solid weekly performance despite the decline in ETF-held BTC, while Sept. 18 stood out for strong one-day inflows across both Bitcoin and Ethereum spot ETFs.