But in that case, the carbon source is used to feed bacteria that break down nitrates in the water column. And if care isn't taken, bacterial blooms get out of control.
To be sure alcohol will pickle your fish (but may provide "incentive for the hobby?), but the dosage levels are miniscule relative to tank volume in the reefkeeping applications.
If you think that's bad, you never saw the thread that tossed out the idea of urinating in a reef tank to lick start a cycle.