Without pics we are simply Assuming that this is a Columbian Shark, which is exuding classic symtoms of ammonia burn, the small tank doesn't help things much but we have been informed already that there are plans for a larger tank in the near future so lets leave that as it is (clearly this person knows enough about the fish to know that it needs a larger home or they wouldn't have felt as if they needed to tell us this from the beginning), At the size that most of these guys are sold.. (between 1-3 inches) it really isn't going to be a problem for the 3 gallon for week it's been there.cenecker;1446773; said:also - your tank is WAYYYYYYYYYyyyyy too small for that species. Not only do columbian shark cats get really big (well over a foot), they are also very very active. Its likely that your water is toxic but additionally shark cats tend to be very spastic and nuerotic when they don't have enough room to stretch.
Don't feel too bad I think thousands of people take home these poor fish every day having no clue what it takes to care for them long term. They were actually MY first fish! I crammed them in a ten gallon where they were reasonably happy for a month or so. Within a year they were trying to jump out of my 55 because it was just too freakin small for them.
Good luck
Please take the time to clearly read the posts As this type of posting to their threads is not something which is encouraging a young fish keeper, We prefer that they keep coming to a site like MFK and learning about this extremly addicting hobby.