Behavioural change?

  • We are currently upgrading MFK. thanks! -neo
Ahhh I see.... okok he's been separated since then. But till now he is never active as he was before in his community tank..

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Many of the Geophagines (and this includes Acarichthys) are social fish, which like to live in shoals with members of their own species, if kept separately they can become either hyper dominant, or extremely paranoid. Your fish is very young, so it is simply showing signs of dominance.
Acarichthys are actually thought to be on the aggressive side, as far as Geophagines go, especially in any tank under 6ft .

 
Add some dither fish(giant danios, silver dollars). That might perk him up. They are much faster and hard for him to catch.
 
Not to belittle the value of a dither fish but they will adapt... and after awhile the cichlid will give up chasing them
 
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