First, lets's clear up some misunderstanding.  The annual amount of a capital loss that can be taken against other income is $3,000.  That means, if you do not have enough other forms of taxable income to absorb your loss in the year it occurs, you can carry it forward and deduct up to  $3,000 against other tax able income each year.

Your loss was only $700, so any capital gain you had in the same year, would have been reduced by that loss.  If you had no gains, then the $700 would have subtracted from any other form of taxable income you had.  It is not a subtraction from tax owed, but instead reduces your other taxable income.  If you had as much as $700 in taxable income that year, then your loss was already used.

You should look at the 1040 in your 2017 tax return to see if the loss shows on line 13.