Although it does not discuss the difference between ethanol and acetate, some deduction can be made from the statements involving similarly structured substrates used in this study.
Coassimilation of Organic Substrates via the Autotrophic 3-Hydroxypropionate Bi-Cycle in Chloroflexus aurantiacus
If anything, it seems to imply that the pathway used to assimilate the various substrates would process acetate even slower than ethanol, due to the energy held in the double bond. A difference opposite from what the experience of aquarist seems to imply, but a difference none the less
It is difficult to wade through some of the available literature, especially when seeking information regarding ethanol as a source of carbon, not a final metabolic product (i.e, Biofuel synthesis etc.).
Perhaps the research is untapped - an opening for any industrius research students looking to make a name for themselves in aquaria
This article has some interesting information regarding the use of ethanol as a primary source of carbon, but not in the scope of cyanobacteria - it mentions acetate passingly but not in the same depth.
I imagine if I still had the database access I was granted in my days of school the search might be more futile, but all good things must end