This happened to me , and it turned out to be a power head the wire was broken near the Epoxy I noticed it when I looked down and I saw a flash every now and then.
Removed it and have not had it since.
Changes are it could be the power head. Like everyone said unplug various pieces of equipment and test again 
Jim
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Setup: 180G DT, 105G Refuge (approx. 300lbs LR, 150lbs Aragonite)
Hardware: Super Reef Octopus SSS-3000, Tunze ATO, Mag 18 return, 3x MP40W, 2X Koralia 4's Wavemaker
Lighting: 5ft Hamilton Belize Sun (2x250W MH, Blue LEDs) Vertex Calcium Reactor 2x Deltec Pellet Reactors
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