The type of leader you choose whether it be tapered, straight, mono or fluro-carbon is very much a matter of personal preference however whatever you choose, the leader needs to sink.
This is important because a leader on the surface film of the water is very visible to the fish and will certainly result in less takes. It’s also important when nymph fishing that a buoyant leader isn’t stopping a nymph getting to it’s natural fishing depth.
There are plenty of concoctions to help the leader sink out there and every angler has a personal preference. Mine is to mix a liquid sinkant with fullers earth and rub a generous quantity on the leader material.
A leader will always sink better with a ripple on the water and on very still days it may be difficult to get your leader to sink on a pond or reservoir. In these circumstances I normally put a tiny shot on the leader.
It’s not ideal but needs must!