If you intend to spend 3 months or more in Spain in any 12 months annually, then you must register on the EU citizens register. It is not six months. Although Residencia (and the ID card) do not exist, in practical terms registering and getting a residence certificate means you are immediately resident with all the privileges and responsibilities that entails, including making an income tax returns in Spain.
If your principal residence is in Spain then you are deemed ‘fiscally resident’ regardless of the time you spend in the country.
Don’t forget the double-taxation treaties.