Either the tank isnt brackish and they are keeping the puffers and monos in freshwater to save money on salt (which is standard practice for most lfs) or the fish you saw were one of the "shark" catfish of the Arius family such as the Columbian sharkcat Arius seemani which look similar to Pimelodus species but have sorter barbels and wider bodies.
i know the owner well and they are indeed pictus cats, i discussed getting one but i decided n a 5" pictus i found, and when i asked he said the tank was brackish
Then the owner of that store is not only a idiot for keeping non brackish fish in brackish water but cruel as well since Pimelodus pictus orginate from regions where the water has no or very little measurable salt content and so have no means in which to process salts through their bodies. What happens when non brackish/marine fish are kept in salted water is that the salts build up as hard deposits on the kidneys and in time the fish dies of kidney failure which is long slow death as the fish slowly fills up with liquid inside its body as it can no longer process liquids into urine.
Its either that or he/she doesnt actually know what brackish water is, do they realise that brackish water has at least 1 pound of marine salt for every 10 gallons of water?
i will ask him tomorrow again so i know for sure that the GSP has been living in brackish, im just trying to find a tank for the pictus since im more then likley making the 20G brackish, 29G malawi 29G Community 55G Frontosa
If you cant house more than one of them then i wouldnt bother kepping him anyways. Why not try and turn him in for instore credit. Much better then constantly moving him around and keeping him solo.