Algae are weeds in the water garden container. They can be controlled through natural means and through chemicals. Algae need water, sunlight, and nutrients to grow. We can't take away the water, but we can limit the sunlight and nutrient levels. Shade can be created by locating the pot where it doesn't get too much sun and by planting lots of plants in the water garden. Excessive nutrient levels in small containers tend to come from the fertilizer tablets placed in the soil of the plant pots. Make sure such tablets are completely covered with soil.
Underwater plants and floating plants get their nutrients directly from the water and compete directly with algae for nutrients. Floating plants shade the water, too. Potted plants get the majority of their nutrients from the soil in the pots and only compete for sunlight.
There are a few algicides that are safe for other aquatic plants, fish, and the other organisms found in backyard ponds and water garden containers. Algaefix from Aquarium Pharmaceuticals was the first of these products and can be used in aquariums as well.
Mosquitoes only become a problem in water garden containers that are not maintained. If small aquarium fish or goldfish are in the container, they will eat any mosquito larvae. A natural product called Mosquito Dunks contains the bacteria Bacillus thuringiensis subspecies israelensis. It can be purchased inexpensively to completely eliminate mosquito larvae in all landscape situations that can hold water. The product can be placed in gutters, rain barrels, swimming pools, birdbaths, animal watering troughs, and anywhere else water accumulates. This product is effective, long lasting, and safe for people, pets, and fish.
Attempting to overflow the water garden container to get rid of mosquito larvae can be partially effective. Some larvae escape by swimming to the bottom of the container when the water starts to flow. Flooding the container trying to get rid of every last larvae can waste a lot of water.
Adding a small amount of vegetable oil on the water surface can temporarily prevent the larvae from surviving. It can be unsightly to see an oily surface and it coats plants and pots. It doesn't work if there are fish or fountains in the container.
Water garden containers can be filled with gravel after the plants and water have been added. The water will not be visible or available to mosquitoes.