"don't keep them alone, they like to be in groups"

[jjohnwm]

At the size above, all was copaseptic, all cichlids were safe, but when I experimented by adding a 12¨ adult goby, cichlids, plecos and tetras all quickly
disappeared....Same thing happened when I tried adding an adult Macrobracium shrimp...

At least the pH was right.

...and I'll bet the nitrates were undetectable!

Really, not putting magnum-sized predators with bite-size prey items isn't a biotope issue or an aggression issue or a rheophilic vs limnophilic issue...it's just a general aquarium "Don't do that!" issue.