the brackish 'gars' are not true gars as stated above, but needlefish which belong to the family belonidae. if you are looking for a 'gar-like' fish that can stay in a 250g tank, a needlefish would definitely work.
all true gars (including the one pictured) belong to the family lepisosteidae. the longnose gar, although commonly seen in fish stores, can reach around 72" in the wild, and easily 2-3' in captivity. they are great fishes, but require large tanks for keeping them for life, and they can be quite skittish. longnoses easily reach 18" in their first year alone, which would make them too large for an 18" wide tank (at least 24" would be needed at that point, but preferably 3'). --
--solomon
