My fish which spawn do not appear to be affected by lunar cycles. They respond more to weather changes. The fish can sense changes in barometric pressure. This is common for fish which live where there are seasonal changes, that is a dry and a rainy season. When the storms that happen during the onset of a rainy season approach, the is a drop in the barometric pressure. the fish sense this and begin the spawning process even before the rains storms hit.
In many of the places with such seasonal change there is often water which is supplied from mountainous regions.there the onset of warmer weather melts the winter snows and the water goes into the upper ends of the rivers found lower down. So cold water is also flood in at the same time. the combination of the cooler rain and mountain water cause a change in the water temp. as well as how clear it is.
But maybe other fish react more to moon phases. I went llok for research papers and here is an example of what I found.
Desjardins, J.K., Fitzpatrick, J.L., Stiver, K.A., Van Der Kraak, G.J. and Balshine, S., 2011. Lunar and diurnal cycles in reproductive physiology and behavior in a natural population of cooperatively breeding fish. Journal of Zoology, 285(1), pp.66-73.
Abstract
Natural environmental periodicity that occurs on both the small scale like day length, or larger scale like lunar light can provide animals with valuable information about resource availability and predation risk. Such environmental cycles are often linked to the timing of reproduction. Here, using the circulating androgen concentrations, gonadal investment patterns and detailed behavioral observations we show that wild populations of the group-living cichlid, Neolamprologus pulcher from Lake Tanganyika, have marked diurnal differences in behavior and lunar synchronicity in their reproductive physiology and behavior. Female ovarian investment peaked in the first quarter of the lunar cycle. In males, plasma steroid hormone levels and sperm swimming speed were highest at this same lunar stage, supporting the idea that egg laying occurs during this phase and that young will emerge at full moon, perhaps because nocturnal predators can be best detected then. Female subordinate group members' gonadal investment patterns mirrored the lunar pattern observed in dominant female breeders. In contrast, male subordinates did not show a change in gonadal investment or in steroid hormone concentrations across the lunar cycle, suggesting that female subordinates, but not male subordinates, reproduce within the social group. Neolamprologus pulcher demonstrated diurnal cycles in behavior, with higher rates of feeding in the morning. Male and female breeding pairs were strongly size matched potentially as a result of size-assortative mating; also the gonadal investment of male and female mated pairs was strongly correlated indicating within-pair reproductive synchronicity. In general, this study provides evidence for the impact of environmental cues (sunlight and moonlight) on circulating hormones and reproduction in a small tropical freshwater fish.
The majority of papers I found dealt with salt water fish more than anything I found re Fresh water species. But is is clear to me that at least moonlight matters to the spawning habits of some fw fish. But the papers all deal with fish in nature not in aquariums. I read that one reason for fw fish to spawn in response to lunar cycles was that, when the babies became free swimming, moonlight made it easier for them to avoid predation at night.
Since the moon controls tides, it may be the lunar gravitational effect influences fish which live exclusively indoors as the difference in light would not be visible to tank living fish in most places.
