Overflow not maintaining depth

[Illwanted]

I have a 120 gallon tank with dual overflows. One of the overflows is draining down about 8- 10 inches and then filling back up. Is my pump about to go out? This tank has been setup for well over a year and the first time I have had any issues with plumbing. It has not been well maintained over the past month as the tank is in my office and today was the first day I could come in and really look at it. I have had an employee fill the water and feed the fish daily, but nothing beyond that. Thanks in advanced for suggestions.

 

[dank reefer]

Have you checked to see if there is anything obstructing? Maybe a snail got stock in there