"This tea is spicy, isn't it?"
If you used just ですよ it would be something like "Duude, this tea is spicy!" (as in like, introducing information that the hearer doesn't know yet). If you use ですね it sounds more like "wow, this tea is spicy, isn't it?(therefore you're both acquainted with the tea's flavor and you're able to give out an agreement or disagreement answer).
Overall, よ would be for emphasis on new information that the hearer might not know
First of all な is na. Ne is ね.
Secondly, if you just used か, the sentence would be more like "Is this tea spicy?" In contrast, with ね, it's more of "This tea is spicy, right?" The first is just a general question that you'd ask without having even drunk the tea yet. The second is something that you'd say after you've already drank the tea, formed your own opinion on it, and wanted to see if others shared that opinion.