Juveniles of both Colomesus asellus and Colomesus psittacus look essentially the same. The way you tell them apart is to look at the base of the tail: on Colomesus asellus there is a black patch on the underside; on Colomesus psittacus this patch is absent. In practical terms, because this patch often merges with the stripe that goes on top of the base of the tail, Colomesus asellus often looks as if there is a irregular black band running around the base of the tail.
Both Colomesus asellus and Colomesus psittacus live in brackish water; essentially they overlap in ecological range, Colomesus asellus extending from the Amazon river into the top of the estuary, and Colomesus psittacus extending inland from the coastal Atlantic into the Amazon river. Colomesus psittacus does enter freshwater, but likely doesn't spend its entire life there.
In terms of care, Colomesus psittacus likely needs higher salinity conditions for good maintenance. Certainly more than SG 1.005, and probably around 1.010+. Colomesus asellus will do fine up to SG 1.005.
As others have mentioned, it's often the case that Colomesus psittacus is the name given to specimens of Colomesus asellus. Some books still use the name Colomesus psittacus for Colomesus asellus. The differences between these species haven't always been recognised, and there's a great deal of confusion among some retailers and hobbyists. They are different species though, Colomesus psittacus being much larger (up to 30 cm) than Colomesus asellus (around 7 cm).
It isn't known to me what the social behaviour of Colomesus psittacus is in captivity. Colomesus asellus is gregarious and (when not nipping slow-moving fish) is completely peaceful. Whether this holds for Colomesus psittacus I cannot say. Fishbase does mention that Colomesus psittacus is solitary or lives in groups of 2 or 3 specimens, but doesn't form schools.
Cheers, Neale
