It will also depend on the type/number of fish as to how stressful they will find 40% water level fluctuations. If you are stocking as if you have a 2:1 ratio then the number of fish will probably mean that the water level fluctuation would be too much and you would need to mitigate the fluctuation with extra water from a sump or some means of sequencing the flow to the grow beds. However, if you are stocking the fish tank very moderately compared to the volume of your grow beds with a 1:1 ratio of grow beds to fish tank then the fish will probably be ok with the fluctuation.

Extra water in a sump tank to handle the fluctuations is good as long as you are not tempted to stock fish in that tank too. There are also some handy things you can use a sump tank for. If you let your fish tank water flow through grow beds before going to the sump tank, you get a "clean water sump" with water appropriate to go to things like towers, NFT pipes, or even raft beds as well as being pumped back to the fish tank.