If I live in Florida and made all my money in FL but have a rental property in DE with a net loss, do I need to file a DE state tax return?

Yes, You must file a Delaware state tax return if you have any gross income during the tax year from sources in Delaware.If your deductions offset the rental income you are still required to file.

Thanks for the help!

You are welcome, unfortunately, DE is one of a small handful of state that require filing even with a loss.  It becomes a problem for people who did not know and did not file when they sell the property.