They love mosquito wriggler when small guppies and live crickets when larger, they tend to be skittish and seem to prefer living in small shoals. Like a bit of salt but don't require brackish, 1tbsp/5gal is fine. There are several species of needle fish, at least 4 live very well in full FW, others are brackish, and some fully SW.
I could be mistaken but I believe they occur *mostly* in fast-moving rivers (in india) but can be found in brackish conditions. I've seen one that was at least 24" in captivity, at this age and size it's beak was practically GONE. Suggest a VERY BIG aquarium with a strong current, and yeah turn lights on gradually and take all other precautions to keep them from getting spooked and swimming at breakneck speed into the glass!
On a personally note I used to have a pair that would eat freeze dried krill and would attempt to eat pellets (however thier mouths are just not designed to chew...sooooo). But yeah from what I hear that is the exception not the norm.