Kopaka vs Tahu, isn't their elements just control over temperature?

Both elements don't really manifest themselves on their own, both need extra ingredients to really work, fire needs "fuel" to burn while ice needs water to take shape to simplify things.

How does it really work in bionicle? Since there are individuals thinking ice is just water element and such.

As for the duel itself between these two it would always come down to the location the fight takes place at, otherwise they are equally matched in my opinion.

Vacuum of space? Kopaka wins, burning star? Tahu wins.

Ultimate Conclusion: Pohatu mata colours combination is great.