I have 2 Tunze 6100's that have been working fine for about 7 months now. 1 of them just started beeping today, and I can't figure out why. I cleaned out the inside of it, took out the impellar, which looks fine, put it back together and plugged it in and it's still doing it. Any recommendations on how to fix this? I need to get the flow back in my tank. Thanks!