The water level will vary slightly depending on the age of the pump and how much water you have in the system but generally it will sit about an inch of two bellow the first grid plate in newer units (Made from 2015 onwards)
The lower water level allows the skimmer to work at optimal efficacy without poking out the top of the aquarium casing.
On all newer units the heater should be run on the lower shelf.
Variations in water level will occur depending on how much media is packed into the shelf system. As long as the water is not above the top edge of the Fluval tank there is normally no need to adjust water levels!
To increase the water level in the marine-pac 3 you can turn down the flow slightly on the compact 600 pump.
To increase the water level in the marine-pac 3 you can decrease the flow slightly using the flow adjustment on the front of the compact 600 pump
It is also possible to adjust water levels in the display tank and Marine-Pac 3 by adjusting the position of the EHEIM Compact 600 in the Marine-Pac 3. This can he achieved without removing the Marine-Pac by gently pulling on the power cable of the EHEIM Compact 600 to move it higher up in the Marine-Pac 3. Make a small adjustment and wait 30 seconds for the water levels to change before adjusting any further.
You can also fine tune the water level by sliding the inlet outlet along the edge of the aquarium (towards or away from you)
Keep in mind after moving the inlet/outlet it will take up to 40 seconds for the water levels to change.
In most cases the pump will work best if left on full.