Gerbil troubles?

im having a hard time. i have a 5 month old gerbil, ive tried everything to "train" it, usually if you tap a gerbils head when it bites and say "no" theyll learn to stop, its worked with my friends rat and hampster, and all my other gerbils. but this one is diofferent. we saved it from a bad pet store that had the needle in their hands to put it down as we walked in because it was "too old" it was 3 weeks old and they couldnt sell it and they didnt want it. ive tried everything to calm the poor guy down but nothing works, he barely eats, barely drinks, never plays, and does nothing but burries himself and sleeps for days on end, he dosnt even leave his nest to go to the bathroom, he just sits in his own filth. i have him in a 10 gallon well airated fishtank, which is biggger then they tell you to put them in (they say one to a 5 gallon, 2 to a 10 gallon or smaller, so he has plenty of space. if i try to clean the cage, take him out to clean it, or refill his dishes he bites...cont.