I've had a the same problem for the longest time. Last week I did a couple of things and I'm not sure which one exactly fixed it:
- Someone suggested that it might be stray voltage, so I removed my 2 powerheads
- I added fresh carbon
- I used Polyfilter
within a day of doing the above, zoos started oppening back up. I'm not sure which one exactly did the trick. The poly filter turned brown, so it could have been somthing in the water the it took out.

good luck