Get him a small LED light fixture and skip the filter. If he already has a tank, he'd probably rather do the filter stuff on his own anyway. LED lighting is the bomb and the prices have started to come down. In addition, most of what he will likely put in a 5 gallon nano will look super under the LEDs. You could get a very cool LED light fixture for a small tank well within your budget and it would be a most thoughtful gift.