This is what I got from frequently asked Q&A questions.
At the farm in Hawaii, the water in the aquaponic tanks are drained weekly. There is more to it than just nutrients, bacteria and fungal counts will also rise in the water, some of the 'silt' ends up in the bottom of the tanks. Algae grows and takes up oxygen and turns the water green. Some types of algae will actually start to plug the system when it grows.
The hydro system also needs to be flushed periodically and everything needs to be cleaned.
How often should I change my nutrient solution?
There really isn't an amount of time that should be used to determine how often you should change your solution. A good rule of thumb to follow first of all is to top your reservoir off with fresh water without any nutrient added. This is because you will lose water to evaporation and plant uptake but the strength of the solution does not necessarily drop with the level of the water. So, in some cases as the reservoir water level drops the solution concentration actually goes up or gets stronger. So, add only fresh water and then adjust your pH accordingly. Try and keep a record of how much water you are putting in the reservoir to top it off and once the amount of water added equals half of the reservoir capacity it is time to change the solution and rinse the reservoir and growing medium. For example if you have a 20 gallon reservoir and over the course of 12 days you have added 10 gallons of water, then it is time to change your solution.
Happy gardening in Hawaii. Gardens are where people grow.