There are alot of products on the market for killing aphids that sometimes find our plants in the water garden.  I use a mixture of soap and water  to spray mine which seems to work for me rather than purchasing a product on the market.  This is natural and won’t harm your plants, fish or water.

1 Tbl liquid dish soap and 1 cup oil.  Mix this together and then take 1 tsp of this mixture and add it to 1 qt of water and put it into a spray bottle.  You don’t want to do this during the hot sunny day but during the evening or early morning when its cooler.   Its never a good idea to treat any plant during the heat of the day.