Yes you are correct if you are a non resident then you pay tax. It is basically a tax that you pay each year for owning a property in Spain.
I don't know the figure yet for my property on Hacienda as my tax return is not due. What I can tell you is that I own another property similar in cost to el Oasis apartments and off the top of my head the tax is roughly 100 euros per year, might be a little less but seriously it is not a huge tax. My lawyer writes my tax return yearly. If you ask your lawyer to do this for you when you complete they will email you when it's due, complete the return and ensure it is paid from your account. Mine costs 70 euros for the tax return Inc IVA, your lawyer might be cheaper. I have friends that complete their own but I'm not that good,ha!
When you become resident you will not pay this.