Tetras have always been my nemesis...

[Magnum Man]

I've been keeping fish on and off for between 30, and 40 years, living in this same geographical area... with " rock hard" water, so keeping tetras, has always alluded me... adding an RO water system for my tanks, has helped greatly, but for at least another year after adding the RO, they were a huge challenge, but a few were surviving here and there, I'm not trying to breed them, just trying to maintain a tank full of various species... finally I have a thriving tetra tank... I need to move a showy angel fish to a different tank, then who knows what might happen, but I've gotten to watch several species dance... I've had to work at it, but I finally have a nice tank of tetras, they have been one of the hardest of fish for me to keep... what fish have been your nemesis, and did you finally win them over???

 

[Magnum Man]

big school of palmeri telling that angel they want the coveted spot in the center of the tank..

 

[GaryE]

I haven't had issues keeping fish alive, but breeding them has me with a few nemesis species.

Tateurndina ocellicauda - peacock gobies. New aquarists breed them all the time, but I've never managed that in many tries.

Cap lopez killies - Aphyosemion australe. They are supposed to be the easiest of killies to breed, but it took me about 10 pairs over 25 years to finally succeed in raising fry. Meanwhile, the supposedly difficult killies of that group were just fine.

Licorice gouramis - Parosphromenus spp. Here, I have an excuse, because they are genuinely hard. But they have given me great difficulties in half a dozen attempts. I can keep them alive, but they won't show me their full behaviour.

For years, I bred all kinds of rare and undescribed Apistogramma, but I had never successfully bred Convict Cichlid group fish. But once I managed that, they went nuts. I had the closely related Honduran red point in stupid numbers.