Aquatic weed management  
Various kinds of aquatic plants are present in the ponds used for aqua culture. These plants defer vary drastically in their location, size and nature. Some of them are microscopic organisms such as plankton algae which flow with the water. Some of them are rooted in to the bottom of the pond. Some of them are essential for aqua culture but plants those have adverse effect on fish production are considered as weeds. Owner of the fish farm should get read from these weeds as early as possible.
In order to increase fish production many fish farm owners tend to add large amount of commercial food and chemical fertilizers to their ponds. Additional food and fertilizers creates ideal environment for weeds to grow. These weeds hamper harvesting because they don’t allow harvesting seines nets to reach deep enough in the pond, and they block its way. This results in less fish production than what is expected. Harvesting is even more difficult in ponds with dense population of weeds, because if weeds get trapped in the seine net then it becomes impossible to pull it as its weight increases considerably.
If they some how manage to pull out the seine net with additional weight of weed, still it is not the end of problems. It is extremely difficult to differentiate between fish and weed and time consumed in the process is stressful for the fish.
Detailed information about aquatic weeds is described below which might help readers to understand this problem in a better way.
Weeds can be categorized in four groups such as; algae, floating weeds, immersed weeds where parts of the plant used for photosynthesis are above water surface level and submersed weeds where most of the plant is under the water surface level.
Algae are the most common type of weed. Size of this weeds ranges from microscopic organism to full grown plant, which looks similar to submersed weeds. Algae are unable to produce seeds or flowers. Algae is further divided in types such as; plankton algae, filamentous algae and stonewort. Plankton algae produce most of dissolved oxygen in the water which is a lifeline for fish. Most of the oxygen is produced during day time when sunlight is present. Rate of oxygen production is lowest during early morning and highest in the afternoon. Disturbance in this cycle may result in death of the fish.
Having said this, excessive presence of any useful thing also causes problems. As described earlier owners tend to add large amount of commercial food to the pond that may result in excessive growth of algae. At many places commercial food is added to the pond in such a way that doesn’t result in excessive growth of the algae. It is very risky to use artificial ways to thin out this algae growth. But it has been observed that spot treatments of plankton algae scum are effective in many ways.
Some algae produce compounds which are responsible for musty odor in fish flesh.
Fish tend to absorb such compounds which some times causes extremely offensive flavor. Three to ten days required to get read from this condition. To achieve this, fish are transferred to the water that doesn’t contain any of these compounds; there is no other way out. Immersed weeds have their stem and leaves above water surface though they are rooted to the bottom. They are mostly located in the shallow water and on shoreline. The most common example of such algae is water lily. Flowers and seed heads usually stay above the water surface in most of these algae, for example hydrilla.   
 
Measure to be taken to control the growth of weeds:
Proper construction of the pond and its maintenance helps to control weeds. Biological methods such as, use of grass carp and aquatic herbicides are also useful. Aquatic weed management largely depends on proper identification of the weed because appropriate preventive measures can only be taken if weed is identified properly. 
Techniques which used to get rid of these weeds are; preventive techniques, chemical techniques, mechanical techniques and biological techniques. Use of any of these techniques depends on type of weed to be eliminated, cost of process, fish production, etc.
Preventive measures
It is always better and less expensive to prevent growth of the weed right from the start rather than controlling it after it grows out of control. First step of prevention is careful selection of location and proper construction of the pond. Edges of the ponds should be deep as it reduces area of shallow water resulting in less space to grow for rooted algae.
To increase fish production capability of any pond measures like farm fertilization are used. Farm fertilization is extremely handy in preventing many weeds from excessive growth. In this process plankton alga grows rapidly because of fertilization, this blocks sunlight and prevents it from reaching to the bottom which reduces the scope for rooted aquatic weeds. Care should be taken that fertilization bloom grows before spring growth of rooted algae. Some algae can be controlled by exposing them to the chilling and dry weather by decreasing water levels.
This works in cold season. But some algae like hydrilla can bare such weather. 
Biological measures
Generally grass carp is used in such methods. It is a natural and economical process of weed control. It is useful in controlling weeds with tender succulent vegetation such as filamentous.
Mechanical methods:
Machines like aquatic weed cutters and harvesters are used in these methods but use of machines in fish ponds is not highly recommended.
Chemical control:
In this process identification of the weed is extremely important. After identification herbicides are used for weed control.
If all the measures mentioned above are combined together, owners can achieve much better results.