Answer: Lilacs sometimes suffer from a mildew infection at this time of year, this can turn the leaves white or silvery. It does not harm the plant, it just looks ugly. So if this is what has happened, it is going to be okay. If the foliage is turning brown it may be due to over or underwatering. This plant needs full sun all day long and a site that is well drained meaning never saturated or sopping wet. The first year the soil should be kept damp like a wrung out sponge, either by rain or by watering as needed. To know if you need to water, dig into the soil with your finger. If it is still damp do not water yet. If it is planted in a low spot or otherwise wet area you may want to transplant it to a drier location.
The green water is caused by excess algae, but even in a healthy pond the water is not crystal clear. Using the plants as you have should help. If you have too many fish, they may be providing more fertilizer through their wastes than the plants can handle. Also, sometimes the hot summer weather contributes to excess algae even when things should otherwise be in balance. Since this is a smaller pond, providing a little shade at the hottest part of the day could help.
Most often though it is caused at least in part by overfeeding the fish. The extra fish food works like a fertilizer, so make sure you feed only once a day and no more than they can eat entirely in about three minutes. In the meantime, the hyacinths in particular are great at cleaning up excess nutrients so you are on the right track.
I hope this helps.
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