I had something similiar happen a while back. Checked everything I possibily could, all level were spot on, no stray voltage, I could nto for the life of me figure it out.
Heres what i found. Im sitting in the recliner I have near my tank watchin TV and out fo the corner of my eye I see small grey bubbles. They stopped quickly so I couldnt trace them, but later that night it happened again. Heres what was going on
Salwater was leaking into one fo the heater housings causing a short, but it wasnt enough of a short to trip the GFI and it wasnt releasing any stray voltage into the tank. What it was doing was melting a rubber seal inside the housing of the heater, this was causing smoke, wich was slowly being released into the tank and being blow around by the current.
Once I found the source I removed the heater, did a huge water change and within a couple weeks af regular maintenence tank was pretty much back to normal. Strangest thing I have ever encountered, but may be whats going on with you as well.
