We will need a lot more info on your situation as jayjay said, but these fish are very hard to keep for even the most experienced fish keepers. They can suffer many issues in the aquarium;
a. In their wild habitat they find their food by locating food (small crustaceons and worms etc) in muddy substrate with an electrical organ in their body.
Because of this they generally only accept live foods which will burrow in the substrate for it to find, needless to say the substrate should also be of very soft fine sand or something of similar consistancy to allow to the fish to feed and behave normally. One of the most common causes of death for elephant nose fish is starvation from them going off their food.
b. Elephant nose fish seem particually prone to whitespot when introduced to a new aquarium, unfortunately they cannot handle copper based meds which is bad because that rules out whitespot meds for them, they are generally intolerant of most meds.
c. Elephant nose fish have been known to randomly die even in th ehands of very experienced fish keepers, some people have suggested that tiny stray electrical currents in the aquarium from the equipment interferes which fishs own electrical signals, causing much stress to the fish over time.
For more info on keeping them check out;
http
/fish.mongabay.com/species/Gnathonemus_petersii.html
Personally i see them as one of those fish which look cool in the shop but are best left alone, i havn't heard of anyone keeping these fish alive for more than a year or two yet.
i want to see him swimming round the tank lol. in the shop they looked active. are they usualy active?
