Hello Dorothee,
Avoid bacteria “unknown” to the fish (no other new fish and plants and no water or decoration from other tanks).
I feed my selatanensis only with frozen blackworms and whiteworms and living artemia nauplia. The Sphaerichthys species eat dead food from the ground as well as from the surface, so it is no need to feed living blackworms.
Do not feed too much! Adults need only very small portions of food.
You can trigger the reproduction with blackworms. When I start feeding more frozen blackworms, they start mating/breeding.
Young Sphaerichthys need a lot of food. Do not feed to big things, because they can not breath with (for example) big larvae in the mouth.
Do not keep them too warm. Temperature around 25°C in enough. PH 5-6 and the water as soft as possible. Peat or other humic substances are not necessary.
Greetings
Patrick