A bit off-topic, but to contribute to Helene’s question: I keep my Betta hendra pair in a 16 liter tank with very much floating plants, hanging roots, and leaf litter. Enough coverage for the female to hide if necessary. I have them since November, and the male now guards their third bubble nest/clutch at minimum. Young Betta in all sizes swim around – and this picture was taken today:
I have the strong impression that they’re doing quite well …. even in this small tank ….
And to get back to the main topic: The only paros I have which breed and breed and breed and never had any fry since over a year – are my P. nagyi ……………