The Treasury Money Market Fund Trick: A HYSA Alternative for High-Tax States

Holding a Treasury-only money market fund like FDLXX or TTTXX in a brokerage account, instead of a HYSA, can keep more of your money after tax if you live in a state with meaningful income tax, because Treasury income is exempt from state and local tax and bank interest never is. That's the real mechanism behind a piece of advice that circulates often on r/HighYieldSavings and similar threads: open a cash management account (CMA) at a broker like Fidelity or Merrill, and put your cash in a Treasury fund instead. The advice is directionally right and the underlying math is real, but the how-to usually gets skipped, and skipping it is why some people try this and never actually get the benefit. This piece is the operational half: how to actually set it up, which fund at which broker, and the three things the comment-thread version leaves out.

The account matters less than the advice suggests

The Reddit framing centers on the "cash management account," but that's slightly imprecise. The strategy works in any ordinary brokerage account, a plain taxable account, a CMA, or a joint account. What a CMA adds on top is debit-card and bill-pay functionality, useful if you want the account to double as a spending account, not a requirement for the tax strategy itself. If you don't need to spend directly out of the account, a regular brokerage account does exactly the same job.

What does matter is a distinction the advice usually glosses over: the Treasury-only fund is not the account's automatic core (settlement) position. Take Fidelity's own Cash Management Account: it offers exactly two choices for where uninvested cash automatically sits, the Fidelity Government Money Market Fund (SPAXX) or an FDIC-insured deposit sweep. FDLXX, the Treasury-only fund, isn't one of the two options. That single fact is the difference between the strategy working and someone thinking they set it up while their cash quietly sits in SPAXX or a sweep account earning the ordinary rate.

How to actually set it up

1. Open or use an existing brokerage account. A Fidelity brokerage account or CMA both work for FDLXX; check your own broker's fund lineup if you use someone other than Fidelity or Merrill.
2. Buy the Treasury-only fund manually. FDLXX has no minimum investment, and you can purchase it with cash already in your core position at any time, with no transaction fee. This is a one-time or recurring manual purchase, not a setting you toggle once.
3. Set up an automatic recurring purchase. Because the fund can't be the core position, new deposits will otherwise sit in the lower-yielding core by default. A recurring buy (weekly is common) routes new cash into the Treasury fund instead.
4. Let the broker handle spending automatically. When the core position runs low and you need cash for bill pay, an ACH transfer, a debit card purchase, or a stock buy, Fidelity automatically sells enough of your FDLXX position to cover it. This on-demand sell is exactly why FDLXX doesn't need to be the core: the broker treats it as a standing reserve that liquidates itself as needed.
5. Re-check the fund's qualifying percentage annually. The state-tax exemption isn't guaranteed by the fund's name; see the caveat below.

Which fund, at which broker

FDLXX is the more turnkey pick for most readers: it's Fidelity's own fund, has no minimum, and its qualifying percentage is published annually and easy to find. TTTXX is what several other brokerages, including Merrill, make available to their own account holders as the equivalent option; it carries an "Institutional" share class, which can mean a real minimum outside of a specific broker's platform arrangement, so confirm the number with your own broker rather than assuming Reddit's example matches your account. Either fund does the same underlying job: hold enough government obligations to qualify for the state-tax exemption, which is the entire point of choosing a Treasury-only fund over a broader "government" fund in the first place.

What you give up for the tax break

This isn't a free upgrade over a HYSA, and treating it as one is how people get surprised later.

• FDIC insurance becomes SIPC coverage. A HYSA is a bank deposit with a federal guarantee on the dollar amount, up to $250,000 per depositor per bank. FDLXX and TTTXX are SEC-regulated funds, not deposits, so they carry SIPC protection instead, which covers you if the brokerage itself fails and your assets go missing, not a decline in the fund's own value. No retail Treasury-only fund has ever "broken the buck," but "very low risk" and "federally guaranteed" are still two different things.
• The exemption depends on the fund actually qualifying, every year. California, New York, and Connecticut require a fund to hold at least 50% government obligations at the end of every quarter to pass any exemption through to residents of those states. A genuinely Treasury-only fund like FDLXX has reliably cleared that bar; a broader "government" fund that mixes in repurchase agreements, like SPAXX, has not always. Check the fund's current published percentage before assuming the exemption applies, since a fund's mix can shift year to year.
• It only helps if your state actually taxes income. In Texas, Florida, Washington, and the other no-income-tax states, the entire mechanism is worth zero, and a HYSA's typically higher headline rate wins outright with none of the tradeoffs above. This strategy is specifically for residents of states with meaningful income tax, not a universal upgrade.

For the full after-tax math, the state-by-state break-even table, and a calculator that takes your own numbers, see SwitchWize's companion deep-dive on money market funds versus high-yield savings, which this article assumes as background rather than repeating.

The honest counterargument

For a lot of savers, none of this is worth the added complexity. If you live in a no-tax state, if your balance is comfortably under the FDIC limit and you value not thinking about it, or if you'd rather not track an annual tax-letter percentage to confirm a fund still qualifies, a plain HYSA remains the simpler, fully-insured answer, and simplicity has real value. This strategy earns its complexity specifically for higher-tax-state residents with meaningful cash balances who are already comfortable using a brokerage account for everyday spending.

Methodology

The core-position mechanics (FDLXX not being a Fidelity CMA core option, the manual-purchase and auto-sell-on-demand workaround) are drawn from Fidelity's own account documentation and corroborated by independent coverage of the same workaround. FDLXX's government-obligations percentage is Fidelity's own 2025 tax-year reporting; TTTXX's equivalent figure should be checked against BlackRock's current shareholder tax letter, since we did not independently verify a specific current percentage for it. The after-tax comparison math and state-by-state figures live in the companion deep-dive linked above rather than being reproduced here, to avoid two articles drifting out of sync on the same numbers over time.

How we source this. Account mechanics are drawn from Fidelity's own documentation and independent verification; fund tax-qualification percentages are the funds' own annual shareholder letters. See our methodology and editorial team. We take no payment for organic rankings.