Once you go looking, the aquarium trade offers a surprising spread of mosses, and they are not interchangeable. They differ in shape, speed, fuss and the look they give a scape, so it pays to know one from another before you tie it to your wood. Every one of them is a genuine true moss that clings to hardscape by rhizoids and spreads from fragments, so the growing is much the same across the board; the differences are in habit and temperament.
Java moss
The default, and rightly so. Java moss (Taxiphyllum barbieri) is nearly indestructible, tolerant of low light and a wide temperature range, fast to establish and forgiving of neglect. The trade-off is tidiness: left alone it grows shaggy and shapeless, so it rewards regular trimming to stay presentable. For a beginner, a shrimp tank or a low-tech setup, it is the obvious starting point, and it is usually the cheapest and easiest to find.
Christmas and Taiwan moss
A step up in looks. Christmas moss grows in tiered, drooping fronds that genuinely resemble little fir branches, giving a denser, more deliberate texture than Java. Taiwan moss is similar, with neat layered growth. Both are a little slower and a little more demanding than Java, preferring decent light and gentle, steady flow, and both look superb draped over wood once they thicken. They reward stable water and patience more than Java does, but the finished look is worth the extra care.
Flame and weeping moss
These two earn their keep through their unusual habit. Flame moss grows upward in twisting, vertical columns that really do suggest green flames, which makes it useful for adding height and a sense of movement in a scape where most plants sprawl. Weeping moss does the opposite, trailing downward in soft curtains that look wonderful flowing off a branch or an overhang. Neither is difficult, though both reward stable conditions and look their best given a little more light than Java strictly needs.
Phoenix moss and Fissidens
Phoenix moss (a Fissidens) is a compact, slow-growing, fern-like moss prized for foreground detail and for staying small and tidy without constant trimming. Its close, flat fronds hug the surface, so it works where a shaggier moss would look untidy, such as low over a stone or along the front of a scape. The catch is patience: it grows slowly and costs more, so treat it as fine detailing rather than quick cover.
How they are sold and grown
Aquarium moss usually arrives as a loose portion in a pot or bag, or already grown onto a stainless mesh pad or a length of wood, which saves you the fiddly attaching. Whichever form you buy, the growing is the same: rinse it, pull it apart into thin pieces, and tie or glue those over your hardscape rather than dropping a dense clump into the tank. All of these mosses do well in low to moderate light without added carbon dioxide, though the fussier ones such as Christmas and flame moss reward brighter light and a steady, gentle current with denser and more even growth. Warmer tanks push growth faster but can make the shaggier species leggy, so trim a little and often to hold the shape.
Choosing between them
As a rough guide: pick Java for ease, low light and shrimp; Christmas or Taiwan for a fuller, more textured draped look; flame for vertical accents and structure; weeping for soft trailing curtains off wood; and phoenix for fine, tidy foreground detail. If in doubt, start with Java to learn how aquarium moss behaves, then branch out once you can attach and trim it confidently. They all attach and grow the same way, covered in aquarium mosses and propagating aquarium moss.