Endlers

[Stevens2394]

How do I know if my endlers are happy / what can I do to keep them happy ?

 

[Abaddon]

Generally if they're active, responsive, courting/breeding, eating well and not getting sick or hiding all the time then that's a pretty good indication that everything's OK. Like pretty much all fish, they need lots of swimming space, hiding places, clean water, stable and proper water parametres & temperature, good quality food and as few stressors as possible. I think that about covers it. They're very undemanding critters.
Why do you ask? Are you having any problems with them?