Last year I fished with 15 & 17lb fluorocarbon. I put new line on my reels last week and went with 12lb fluorocarbon. I do like the lighter feeling of the 12lb. My ? to the experienced anglers is will it make a huge difference? The places I fish have some under water grass but nothing too thick

12 should be fine

I fish with 8lb and 10lb Fluoro on all my spinning rods and fish heavy cover. I don't have any problems.

12 is fine for most situations. It's a little stretchier than 15 and 17, but you'll only really notice if you try to set the hook hard on a jig or worm from 30 or more yards away. I have 12 lb fluoro on everything but my spinning reel (braid with 8 lb fluoro leader) and pitching rods (20 lb fluoro or braid).

i used 12lb no matter what, unless im jig fishing. then 15lb

Depends on what type of lures you're fishing.If you're fishing a trick worm ,dropshot or a weightless senko the lighter the line you use the more natural the lures will appear yes it definitely makes a difference. Lures will also fall at a different rate with lighter line.Crankbaits will go deeper and have more action with lighter line. Where as heavier line or braid (which can sometimes be the best of both worlds lighter and more sensitive and stronger) can be helpful in wrestling big bass out of grass, timber or trees. Fishing a C- rig or flipping a jig into heavy cover are just 2 instances where you want the heaviest line you can get away with.Fish the same lures on different rods and reels with different line sizes to determine what works best for you. Tightlines

I use 12lb fluoro on all my reels except my flipping sticks which are spooled with 20lb fluoro

I don't own anything less than 15# I either use 30-50 pound braid, sometimes with a leader depending on what I'm trying to accomplish other times 20# mono and occasionally I'll go as high as 30# Big Game when fishing big spinnerbaits in mesquite trees on Falcon or in Mexico.I fish for large fish in extremely heavy cover and the lighter lines are a recipe for disaster. My spinning rods are spooled with 30# braid with 15# leaders.

If you are using a variety of lures it does. Try 15 lb fluoro on a crankbait and then 12 lb and you will see the difference. If you want an even more dramatic effect on how line affects lures switch to mono with a crankbait and take note of the difference. Can mean several feet in running depth changes.