There really is no optimum amount of balls in the tower. If you're not hearing anything when you open that cabinet, something's wrong. Mine sounds like a hurricane 15 feet away. I've spoken to Gary (patent owner and builder of these skimmers) and he's told me that you should have as many bioballs in the tower as possible, but not enough to stop air suction. The way he explained it to me is that the bioball tower is the skimmer. Water and air mix here and cause foam to push down into the box, where it then starts collecting and bubbling up into the neck and collection cup. If the water level is too high in both the mixing tower and the bubble riser, then foam production can decrease and has a hard time pushing its way to the tower. You can also risk an overflow.
If it were me, I'd open up the top of the towers and stick a ruler down there. Many (including Gary) state that a minimum of six inch clearance is needed between the injector and the top bioball. This is to allow a proper "buffer" zone of the water backing up into the tower and lowering air suction. This can cause the skimmer to surge and never reach a consistent foam head. This happened to me. I thought he was crazy, but I tried it and it worked. In your case, it's two towers to worry about. If you have less than six inches, remove enough balls to reach this desired 6 inch measurement. After 24 hours, if that hasn't worked, open the gate valve to allow the water level in the skimmer box to drop a bit. No more than a quarter turn. Is there any Kalkwasser being dripped into the sump? If so, this affects the bioballs. The high pressure and coating nature of Kalk can strip the balls of their sharp, fine edges. Many replace their bioballs yearly and it's like a brand new skimmer again. If you need to talk to Gary at AETech, he's always the one who picks up the phone when I call. Who better to talk to than the guy who designed and built it? His number is 845-227-2812.
Hope this helps.
