A livebearer's tank is a nursery by default
Keep male and female guppies together and the tank's central fact is fry: a female drops a brood every few weeks, and every adult in the tank, parents included, regards a newborn guppy as food. Whether any given brood survives is almost entirely a planting question. In open water, almost none do; in dense, fine planting, a steady trickle make it, which over months is how a pair becomes a colony. So plant choice for guppies is not primarily about looks — it is about engineering hiding volume, at the right heights, in plants fine-textured enough that a two-day-old fry can vanish into them while an adult cannot follow.
Fine plants that hide fry
Java moss is the fry refuge of first resort: a deep cushion of it at the base of the tank is where wild-caught numbers of fry ride out their first week. Guppy grass — the soft, chaotic stems of najas — is the other classic, so associated with the fish it carries their name; left to tangle loosely it makes a three-dimensional maze fry thread through and adults patrol around. Any fine, feathery, fast plant does similar work. The texture is the point: coarse plants offer hiding places an adult guppy can also enter, which is no hiding place at all. Plant these refuges generously and in at least two separate clumps, so a brood is never betting everything on one thicket.
Floating cover for surface fish
Guppies live high in the water, and the fry are born near the surface, so the top layer of planting matters as much as the bottom. Floating plants with long, trailing roots — water lettuce, frogbit (Limnobium laevigatum) — hang a root curtain that fry slip into the moment they are born, and the shade beneath settles the adults, who feed and display more confidently under partial cover. Keep gaps in the canopy for feeding and light. A tank that runs moss below and floaters above has its nursery at both storeys, and broods born at either level find cover within a body length — which, for a fry, is the whole game.
Fast growers as the colony's water polishers
A breeding guppy colony is a heavy bioload in a small box: many mouths, much waste, water that wants to slide. Fast-growing plants are the counterweight, absorbing ammonia and nitrate at a rate slow plants cannot match. Hornwort, guppy grass and vigorous stems convert fish waste into plant mass, and every handful of trimmings you net out is nitrogen physically exported from the system. This is the one tank where rampant growth is a feature — the plants are a living filter running alongside the mechanical one. Prune little and often rather than rarely and drastically, so the fry refuges are never demolished in a single afternoon's tidying.
Balancing cover with display
The keeper's tension is that the planting which saves fry also hides the fish you keep guppies for. Solve it with geography: thickets at the back and ends, a planted wall on one side, and open water kept deliberately at the front, where feeding brings the colony out to show itself. Males display where the food and the light are, so an open front arena loses you nothing. Accept that the back of the tank belongs to the nursery and will look gloriously unkempt. When the next brood is due and you are weighing moss against floaters for that tank, put a follow-up question to KinsLe's Kimmy; she answers using the KinsLe Answers library.