Deductions & credits

Assuming you are asking about taking a medical deduction on your personal tax return:

See https://www.irs.gov/pub/irs-pdf/p502.pdf page 6

"You can include in medical expenses amounts you pay for
special equipment installed in a home, or for improvements,
if their main purpose is medical care for you, your
spouse, or your dependent. The cost of permanent improvements
that increase the value of your property may
be partly included as a medical expense. The cost of the
improvement is reduced by the increase in the value of
your property. The difference is a medical expense. If the
value of your property isn't increased by the improvement,
the entire cost is included as a medical expense."

So if the spa increased the value of your home then you cannot deduct that part.

Medical deductions are an itemized deduction.

Only if all itemized deductions exceed the standard deduction will it be of benefit.

For 2018 the standard deduction is:

$12,000 Single
$18,000 Head of Household
$24,000 Married Jointly

Add an additional $1,300 for over age 65 of blind
Not all itemized deductions count the full amount. Medical expenses are reduced by 7.5% of AGI so if your AGI is $30,000, for example, then only medical expenses more than $2,250 would be an itemized deduction.

It is doubtful that you would be able to itemize at all with only $13K of taxable income or itemizing would be of any benefit.