Lowered electrolytes trigger spawning
Lowered electrolytes triggering spawning: the "Kirschbaum method". In the 1990s experiments showed that environmental cues for gonad maturation are connected to decrease in electrical conductivity rather than to low pH values themselves (though the two are linked), and to increases in water depth, all tokens of a rainy season. Dr. Frank Kirschbaum, of the Leibnitz-Institut, Berlin, experimented with the gymnotiform Rhamphichthys, some mormyrids and African schilbeid catfishes, and others, and reported his findings to the IX (1997) meeting of the Neotropical Ichthyological Association, who posted an abstract of his paper, "New aspects concerning the cyclical reproduction of tropical freshwater fishes" (scroll down at that site for it). A follow-up article by Professor Kirschbaum and others is "Environmental control of cyclical reproduction of tropical freshwater fish", available in pdf form through the hyperlink.
Amateur fishkeepers became aware of this role of reduced conductivity, with increased water depth and lowered temperatures, effected by incremental additions of very soft water, when Dr. Kirschbaum published a bombshell article in Heiko Bleher's quarterly magazine Aqua Geographia no. 20 (2000) under the title "The breeding of tropical freshwater fishes through experimental variation of exogenous parameters." Within a year, hip Germans cued to aquatic newsgroups like de.rec.tiere.aquaristik were refering to the "Kirschbaum method" for unlocking natural hormonal spawning keys in the kinds of fishes that are currently only forcibly spawned in Southeast Asia through hormone injections. Corydoras species are notably encouraged to spawn by a 75% water change with cooler, softer water.