i'd talk to him and see if he or his parents are willing to buy a bigger tank, he really needs to increase the number of tiger barbs to six but i wouldn't tell him to do so in that size tank. have you noticed if the neons, the glolights, and the danios are schooling together? if not then he needs to increase those schools too (again only tell him to do so if he gets a bigger tank).
tell him to only put in the amount of food that his fish can eat within a minute, and a
water change with prime or stress coat + every week like amanda said.
you can also direct him to this site
