problem is most US natives outgrow even a 180... but.. imo a brown bullhead may be a nice option ( imo they stay small enough to not threaten a Gar, 'nore small enough to be gar food. but it's still possible.) or a small group of the more docile panfish ( some have some pretty intense aggression, so be wary. and do your research and prolly read some threads in the Native forum as well) . LG and SM Bass are to active and imo get on the "almost to big" size for even a 180, particularly with other tank-mates. Def read the suggested thread as well.