What will happen if you have too high water level in the growbed?
Initially high water level will cause higher evaporation and water will be use more due to this. To ensure this don't happen it is recommended that the water level is at least 1 inch below the hydroton or media level.
Above photo the water level is about 1 inch below the hydroton and it is just nice for most setup.
What else could happen?
Algae and some time moss will grow, moss could turn into fern in some cases. Algae and mosses will compete with plants for nutrient and it also will make the growbed looks dirty as below.
As a general rule 1 inch water below the media is acceptable, this is the DryZone of an aquaponics growbed. This DryZone can be thicker depending on type of plants grown.
A siphon with adjustable standpipe height is helpful in ensuring this water level is easily achieved.