Is the initial £20,000 investment through the Fluid ISA tax-free?

Tax treatment depends on the individual circumstances of each investor and may be subject to change in future.

Yes, as an IF ISA, the initial £20,000 investment is treated like any other ISA held with a bank for tax purposes, and as such is not subject to tax. Please note that your £20,000 allowance is an annual limit and is shared between all ISAs you hold.