What is the most difficult to keep species you have kept? Mine by far have been the crystal red shrimp I have currently. Temp, ph, nitrate levels, WC..... any of these slip and you lose fast!
Needle fish have been the toughest for me because they will only eat live food IME and getting them clean live food is tough when I run out of my own feeders.
Altogether though toughest fish I kept would be red volitans lionfish and clown trigger.