ALL plants are helpful. Irises, sedges, reeds, arrow plant, lizard tail, bog bean, obedient plant, water lilies, floating hearts, parrot's feather, forget-me-nots, creeping Jenny, Joe-pye weed, cattails, water hawthorn, swamp milkweed - all are growing in my pond and probably more that I've forgotten right now. Anything that will grow with it's roots in the water is going to help your situation immensely. Basically you want plants that will compete with the algae for nutrients and choke the algae out - no more killing the algae because it just can't grow. You should expect and welcome a nice carpet of algae on the liner or rocks in your pond - that's part of the natural filtration system. But floating algae or string algae, while not harmful, isn't necessarily what you're looking for in a garden pond. But remember - that algae is telling you something. Your pond is high in nutrients. Until you address that, you will continue to have algae overgrowth.