It's not a big deal, really. I'm at the point with my pond (and most of my garden to be honest) that if one thing dies, it just makes room for something else. I don't fight too hard to keep anything alive - I'll give them good soil and plenty of water and the rest is up to the plants. And honestly that clump of irises was getting rather large. Nature is just giving me a little help getting rid of it!
I've always just used a good blast of water to knock down aphids. Obviously ladybugs love them and I know people who buy ladybugs and release them into the garden, but I've never had a big enough issue with them that it got to that point. Marigolds do seem to work, as will many fragrant herbs like cilantro or fennel. Plants in the garlic family also work to keep them away. You can also plant "trap plants" - sunflowers are a good one. Aphids love them, but they can't really hurt them as they are so big. Just keep them away from your susceptible plants.