I have two drains in my tank, and one of them was used to feed the skimmer, while the other is used only as a backup, in case the one feeding the skimmer became obstructed. For some reason, the drain feeding the skimmer, would cause the water level in the skimmer neck to vary up to 8" over a 24 hour period. It was at a constant siphon, so I'm not sure why the problem occurred. Feeding the skimmer with a pump has cured the problem, and I ended up using the main drain from the display to feed my fuge, which will be used for nutrient export.