Flurocarbon is for leaders/traces. Refractive index is close to water making it less visible under water so in theory you can use heavier trace with no disadvantage.
Don't know about stretch as you will only ever use short lengths of it so irrelevant really. I use it on all my light gear (up to about 20lb bs) combined with much stronger braid mainline. I buy it off Ali express
https://www.aliexpress.com/item/SeaK...311.0.0.xv3DOm
for a fraction of the price you can get it here. It does vary brand to brand but you have to try it to find out. The only bad one I have tried was Berkley Vanish (crap knot strength) and that was a long time ago.
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