Plants - the best helpers there are
Pond plants not only look beautiful, they also help your pond to work biologically and ensure good water values. Even if a lot of fish just eat them, you ought to keep plants in your pond.
If you can keep plants in your pond, whether it’s in the water, under water or in the marsh area, do! Plants help you to keep the water clear and to control algae. Plants need nitrogens (ammonium & nitrate) and phosphates (PO4) and absorb these substances from the pond water. Algae are also plants and require the same nutrition. But once your plants start growing strong, the algae begin to starve and can’t grow properly! Algae control can be so easy!
Plant killers: your fish!
The only problem is your fish’s healthy appetite for greens. If you just insert a few underwater plants, you’ll only have a short time to enjoy (and derive benefit from) them. The goldfish and koi will find and destroy them completely. Larger numbers of plants mean a better chance of the plants growing quicker than they can be eaten. The plants in the marsh zone will have a significantly better chance to survive. Although they are in the water, they’ll grow above the water surface. The fish would need to jump off the water to reach them! All that matters here is that the plant roots are in pond water so that they can access its nutrients.
Dead plants = nutrition for algae
You need to remove dying plants, such as water lilies in autumn, from the pond immediately. Otherwise the nutrients, absorbed during the growth process, will return to the water to fertilise the algae.