Like all plants it needs some light and ammonia or nitrogen based food. So do put some fish in with it. :) I realize that a 2.5 is only room for a handful of small fry or maybe two adults if the tank is started with gravel, water and a sponge filter from an established aquarium.

They are wonderful hiding places for fry and chased adults. In fact they are also favorite "ammonia sponges" with a few fry, in tanks as small as 2.5s, but they do need at least good overhead light to use any of that ammonia..

Evidently the hobby has ascribed the wrong scientific name to Java moss. See
http://www.aquamoss.net/Introduction.htm
which I recommended here a few months back. That article also hints at what will, in time, become available in the hobby.

Medicinal dyes (especially acriflavin for velvet) can kill Java moss if it is left with it too long. Likewise it can starve in water with no nutrients or light. I suppose really funky water could do even it in.

I need to learn to pull it apart and give it more space in tanks where it is growing well.  Check to see if the stuff on the bottom of the pile is thriving. If it is dying, spread it out, give it more light.

The most amazing batch of Java moss I ever saw was in a 125-gallon tank kept by a guy into raising aquatic plants. He had a high-powered light hanging over the water, close enough to bounce most of the light into the aquarium, but there was space for the heat to dissipate. He had a CO2 unit feeding a measured amount of carbon dioxide into the flow of a powerhead, which was aimed across the tank and down a little, so that the CO2 wouldn't be bubbled out the surface.

The Java moss, especially near the surface, was full of clear bubbles. This "pearling" indicated that photosynthesis was taking place very quickly. "A pearling plant is a happy plant." ;)

A lot of the tank bottom was covered by one of two species of Cryptocorynes. The rest was occupied by Java moss, which billowed up and out of the aquarium! He had several threadfin rainbowfish (Iriatherina werneri) which were doing fine. He had also put a couple of pairs of the king's tetra ( Inpaichthys kerri) in there. They had become thirty tetras!

While they are pretty accommodating, as tetras go (even I have had a few grow up in a single species tank), I was amazing by how many eggs and fry must have survived in that tank. And they also survived the quite different rainbowfish, who would find them a great snack!