Hey, firstly have you cycled the new tank? or used mature media to seed filter? This will help us to answer your question
James


one green spotted puffer,and a catfishNot quite that easy chaps and chapesses. It also depends on what kind of fish you have.
Always introduce the non-territorial fish first so that they get used to their new environment before the territorial fish get added. This way the non-territorial fish can retreat to known hiding places if fighting kicks off.
What fish do you have?