CKs are pretty mellow for the most part and I think it would be fine with a Gar. The ID Sharks are pretty active swimmers and just don't really mix too well with more calmer fish.
Ye i have a 20"+ clown knife that i have had for about 2 years now when i got it it was about 12" and it has allways been very calm and friendly but very recently it has started to become more assertive especially at feeding time. So i would agree with the thought that clown knifes get more aggressive after they reach the 20-24" mark.
I also think that the ID will be too active especially at night when the gar will be very disturbed by its constant rapid movement.
Are you able to get a larger tank? Because even a smaller species like a florida wil need a 210G for life or something similar. An 80G is far too small for life.
yup, should be just fine for a year...how wide is the tank exactly? depending on that the tank may last for longer than a year.
to Tom500 - what dimension 210g tank are you suggesting? a 180g tank (2' wide) will house most FL gars for the majority of their lives, if not entire lives in captivity in many cases...a standard 210g is just a taller 180g.--
--solomon