I agree with Gary lights out has a short term effect on visible cyanobacteria. It stresses it but it can rebound quickly if husbandy conditions don't change particulary PO4 reduction Light depravation stresses other photosynthetic organisms too perhaps more so.
There are many ways to reduce PO4, cleaning : siphoning, rinsing foods, cut backs in feeding, running a phosphate reducer and organic carbon dosing to name a few. Geting it to low levels will abate cyano.
Living things need 3 main nutrient sources; organic carbon, fixed nitrogen( such as ammonia, nitrite, nitrate) and phosphorous( inorganic or in some cases inorganic) .Cyanobacteria is extremley opportunistic in meeting these needs . It usually needs only , CO2, water and light to survive as it can fix it's own nitrogen from N2 gas which makes up about 70 of the air and makes it's own organic carbon from CO2 via photosynthesis.In the right conditions it can even take up organic carbon in lieu of producing it via photosynthesis but this seems rare. As far as I know it doesn't make phosphorous and needs to scavenge it.
Vodka if dosed properly with good export for organics in play does not cause cyano . I've been dosing for over two years and visible cyano is a very rare nearly non existent and small occurence. The dosing helps keep NO3 and PO4 low along with other methods I employ.
However, in some cases during the early weeks of ethanol dosing some patches of cyano appear in some systems. I suspect it is fed by bacterial by products including CO2 and perhaps some freeing of PO4 from surfaces . I think this may occur as the various bacterial strains get into equilibrium and a balanced cascade with the new carbon source. Using some vinegar in lieu of some of the vodka seems to get the bacterial balance quicker. IME, switching from exclusively vodka to a mix with vinegar eliminated the patchy cyano I was getting in on one frag tank's sand bed.
Think about it chloroplasts in zooxanthelae are closely related to cyanobacteria which may be more sensitive to light depravation than cyano. Other seen or unseen less hardy organisms in the tanks flora and fuana may also suffer more from light depravation than the cyano.
As for the tang hit with ich during lights out consider that crpytocaryon if it's in the tank usually leaves the fish at night with much of it attaching to surfaces near the fish's sleeping station. It also "hatches" from cysts at night mostly ; extending the dark gives it an advantage and may accelerate the life cycle. In these conditions it might be enough to spark an outbreak.
I would siphon vigorously and then set about knocking out PO4 and the cyano will fade away.
I'll have to see if I can find an earlier shot with it on the sandbed/rocks.
