Regarding fertility of certain problem fishes:
kunzi, batesii, herzogi, boctleri and so forth, I'd like to suggest a hypothesis that might explain why these fish are ok to breed in Germany and the Netherlands, but often give others great trouble: Selenium, or rather a lack of it.
1) Any good veterinarian book wil point out troubles in reproduction are almost always too high or too low levels of selenium.
2) The above named fish are from areas rather high in selenium. Concentrations of Selenium in Africa are spotty, about half it it has little. But where it does occur, there can be quite a lot of it. In fact Senegal is the worlds second largest Selenium reserve, as there is a mile thick crust of Selenium phosphate near the coast and only Bolivia has more. It's abundant throughout the Congo, coastal west Africa, Gabon... all the places klillies are found. Where it isn't... well, there's less killies there.
3) Germany has good selenium levels on a widespread basis, there seem to be no areas that are deficient. Netherlands is rather similar. Also parts of Scotland have good amounts, and at least one person there, Steven McDonald, has been reporting on Facebook that he's had no trouble breeding
kuzi and
batesii.
On the other hand some get NO fertile eggs. In the late 1980's, Steffen Hellner sent a pair of kunzi to Kit Stowell in Southern California. Kit got 4 fertie eggs and could not raise them. Hellner had had no problems with them. If Kit was not a highly experienced breeder, I'd think he did something wrong, but I do not. You hear this story over and over.
So, this may not be the right answer but I'd first rule this out as possibility.
Now, to raise the selenium level in fish you cannot just add it to the water, it won't be absorbed this way. You have to feed it to them. In nature algae takes up thre SE, then aquatic animals reat them and so on and so forth as it's passed up the food chain.
It takes 12 weeks for fish to bio-accumulate sufficient Selenium (Se) levels. There's no short cut here. Selenium is used in more than a dozen places in the body and it allows the formation of some pretty exotic compounds, which in turn do the same thing. Measurements in animals big and small all find the same thig it takes about 13 weeks on a high Se diet to show appropriately high levels of Glutathione peroxidase III (GPX3), whicy his a marketr than shows the Selenium is working. The body cannot makes GPx3 without Se, becuase Selenomethione is used as a catylaist in the synthesis of GPx3, which is present in all animal life.
It's also worth noting bloodworms are high in selenium,, one of the few fish live food sources that does bio-accumulate Se. Of course if they're from an area of low selenium, they, they're next to useless for up-regulating Se. Instead one could feed Brazil nut paste incorporated into food, Brazil nuts having the highest concentration of Se (other than seal liver, which can be hard to get, even here in Canada
of any dietary source.
For large scale operations , one could use Watley's formula and ard add half dozen Selenimethione pills from the pharmacy vitamin aisle. Wattley's formula van be found here:
Selenized Wattley's Formula
(use instead of Gordon's formula)
Frozen raw Beef heart, salmon and krill used by US discus breeder Jack Wattley.
1 beef heart 2.5 pounds+, with fat
2 pounds Of fresh Salmon
10 ounces of Spinach
8 cloves of Garlic
6 ounces of Krill Meal
6 Multi-vitamin pills
6 Multi-mineral pills
6 200 mcg selenium pills (Selenomethione or Selenocysteine, not Sodium Selenite or selenized yeast)
Soak vitamins in enough water to cover them or grind in a mortar.
De-fat the heart, grind in a grinder.
Grind the salmon.
Mince the garlic finely. Best to use a garlic press so there are no big pieces of it.
Transfer to food processor, add krill, and spinach mix until a firm paste.
Put into freezer bags, flat.
Break a piece off when frozen and put it in the tank.
Does not keep in the fridge, use frozen.
N.B. I have a lot of references for selenium chemistry and applications in biology and medicine, but didn't include them here as they're rather lengthy. If anybody is interested, just ask,
