Willow moss (Fontinalis antipyretica) is an excellent British native oxygenating plant also good for spawning wildlife as the eggs can easily stick to the small scale-like leaves. It grows well in shade or part sun and is evergreen and provides good cover for pond wildlife, especially newts and dragon fly larvae.
Willow moss is a free floating oxygenator which like Hornwort has no roots so does not need planting. Supplied as lead-weighted bunches as is the industry norm which allows you to ‘fix’ it in one place. An alternative to the lead weighting is to hold it down with a stone or under a planting basket. If you don’t want to ‘fix’ it in place just let it float and find its own depth.