Key takeaways
- Light demand belongs to the substrate reading, not the surface, which is why tall stems and carpets behave so differently in the same tank.
- Anubias, Java fern, and mosses are damaged by too much light far more often than by too little.
- Amazon sword fails from missing root tabs at least as often as it fails from weak lighting.
- An underlit carpet grows upward and rots at the base, and extending the photoperiod makes that worse rather than better.
- Substituting down the table beats fighting a fixture you cannot change, and floating plants let you shade a bright tank deliberately.
Monte Carlo melts. That is the most common planted tank complaint I hear, and the plant is almost never the problem.
It melts because a carpet that wants somewhere near 40 to 60 PAR at the substrate got planted at the bottom of a 50 cm deep tank under a $30 LED strip, with no injected carbon dioxide and a ten hour photoperiod. The plant did exactly what it was always going to do.
What follows is a working reference. Species, the light band each one needs, whether carbon injection is realistically required, and the specific way each fails when it does not get what it wants.
How to read the bands
Light demand is measured as PAR at the substrate, not at the surface and not on the fixture’s box. A tank can be low light at the bottom and high light at the top, and tall stem plants exploit exactly that.
Three bands cover almost everything sold in shops. Low sits at 15 to 30 PAR, medium at 30 to 50, and high at 50 to 80, with the boundaries soft by 5 to 10 units either way depending on who you ask.
If you have never measured, work from your fixture and depth rather than guessing, using how PAR is measured and where it falls off. Guessing a band wrong by one step is the difference between a carpet and a bill.
Plant species against light demand
Botanical names are given because shop labels are unreliable. Several of these species are sold under three or four trade names, and some are sold as entirely different genera.
| Species | Band | Substrate PAR | Carbon dioxide | Notes |
|---|---|---|---|---|
| Anubias barteri var. nana | Low | 15 to 30 | Not needed | Rhizome must sit above the substrate or it rots. Green spot algae on leaves under strong light. |
| Java fern, Microsorum pteropus | Low | 15 to 30 | Not needed | Tie to wood or rock. Browns and develops black spots when overlit. |
| Java moss, Taxiphyllum barbieri | Low | 15 to 30 | Not needed | Grows denser and tighter with more light. Traps detritus without flow. |
| Bucephalandra species | Low to medium | 15 to 40 | Optional | Slow. Also a rhizome plant, treat like Anubias. |
| Cryptocoryne wendtii | Low to medium | 20 to 40 | Optional | Melts after any move or parameter swing, regrows from the crown in 3 to 4 weeks. |
| Vallisneria species | Low to medium | 20 to 40 | Optional | Spreads by runners. Sensitive to some liquid carbon products. |
| Amazon sword, Echinodorus grisebachii | Medium | 30 to 50 | Optional | Heavy root feeder. Needs root tabs every 3 to 4 months or older leaves yellow. |
| Ludwigia repens | Medium | 30 to 50 | Helpful | Stays green at the low end, reddens with higher light and available iron. |
| Rotala rotundifolia | Medium to high | 35 to 60 | Recommended | Survives low light but goes leggy with long internodes and stays plain green. |
| Monte Carlo, Micranthemum tweediei | Medium to high | 40 to 60 | Strongly recommended | Grows upward instead of spreading when underlit, then melts at the base. |
| Dwarf hairgrass, Eleocharis parvula | Medium to high | 40 to 60 | Strongly recommended | Blades lengthen and thin without enough light. Slow to spread for the first month. |
| Rotala macrandra | High | 60 to 90 | Required | Unforgiving. Drops leaves within days of a missed dose or a gas interruption. |
Low light in practice
Low light planting is the most reliable setup in the hobby and the least fashionable. Anubias, Java fern, Bucephalandra, and mosses are all epiphytes, meaning they attach to hardscape rather than root in substrate.
That has a practical benefit beyond light. You can rescape without disturbing root systems, and you do not need an expensive aquasoil, so plain inert gravel works fine.
The failure mode is the opposite of what people expect. These species get damaged by too much light, not too little, and Anubias leaves covered in green spot algae are almost always sitting under a fixture meant for a carpet.
Never bury the rhizome on Anubias, Java fern, or Bucephalandra. The thick horizontal stem must stay in the water column. Buried rhizomes rot within a few weeks and the plant collapses.
Medium light in practice
Medium is where most well kept home tanks actually sit, and it covers the widest species list. Cryptocoryne, swords, Vallisneria, and Ludwigia all thrive between 30 and 50 PAR without gas injection.
Growth is steady rather than fast, which is the point. A medium light tank forgives a missed water change and does not need trimming every ten days.
Cryptocoryne melt catches out every new keeper. Newly planted crypts frequently drop every leaf within a fortnight, and the correct response is to leave the roots alone and wait, because new growth appears from the crown in three to four weeks.
High light in practice
High light is a commitment to a routine, not a purchase. Above roughly 50 PAR at the substrate, plants consume carbon and nutrients faster than a normal tank replaces them.
That means pressurised injection, daily or near daily dosing, and water changes of 40 to 50 percent weekly. Skip any of the three for a fortnight and you get an algae outbreak rather than a slow decline.
The equipment side is not optional at this level, and a regulator, solenoid, and diffuser setup runs somewhere around $150 to $350 before the cylinder. What that installation involves is covered in setting up injection properly on a planted tank.
Carpets deserve their own warning
Carpeting plants are the usual reason someone buys a stronger fixture, and they are the least forgiving group in the table. They grow at the substrate, which is the darkest point in the tank, and they grow horizontally only when light is sufficient.
Underlit carpets do something specific and diagnostic. Monte Carlo and dwarf hairgrass both grow vertically, reaching for the surface, and the shaded base then rots and detaches in sheets.
If your carpet is climbing rather than spreading, the answer is more light at the substrate or a different plant. Adding fertiliser to an underlit carpet feeds the algae growing on it.
Do not fix an underlit carpet by extending the photoperiod. Twelve hours at 25 PAR grows less carpet and considerably more algae than eight hours at 45 PAR. Intensity and duration are not interchangeable.
Species that get filed wrong
Amazon sword is sold as a beginner plant and is often the first thing to fail. It is a medium light plant with a heavy root appetite, and in inert gravel with no root tabs it will yellow from the oldest leaves inward regardless of the fixture.
Rotala rotundifolia is the reverse case. It is listed as high light almost everywhere but survives happily at 35 PAR, it simply stretches, stays green rather than colouring up, and needs trimming into shape.
Anubias is often sold as indestructible, which it nearly is, with one exception. Under 60 PAR and above, the older leaves accumulate green spot algae faster than the plant produces new ones, and the specimen slowly loses.
When the light cannot change
Renting, a fixed hood, or a tank too deep for the budget all mean the light is what it is. Substituting down the table is a better plan than fighting it.
- Wanted Monte Carlo, got low light: use Marsilea hirsuta or Java moss on a mesh mat
- Wanted dwarf hairgrass: use Cryptocoryne parva as a slow foreground
- Wanted red stems: use Cryptocoryne wendtii bronze for colour without the demand
- Wanted a fast growing background: use Hygrophila polysperma or Vallisneria
- Wanted Rotala colour: accept green Rotala and trim it hard for density
Floating plants are the other tool. Amazon frogbit or red root floaters cut substrate PAR substantially and let a bright fixture run over shade tolerant planting, though they need thinning weekly or they take the whole surface.
Reading the failure
Plants report their own light problems clearly once you know the vocabulary. Long gaps between leaf nodes and pale new growth mean too little, while algae on older leaves with healthy new growth means too much.
Holes in older sword leaves usually mean potassium, not light. Yellowing between the veins while the veins stay green points at iron, and neither is fixed by a fixture change.
Tropica publish a light and difficulty rating against every species they grow at Tropica, which is the fastest cross check before buying. Match that rating against your measured substrate PAR and your daily hours, using the run times that suit each light level, and most of the guesswork disappears.
Buy the plant your tank can actually keep. The full set of decisions behind that sits in how aquarium lighting is specified and chosen, and the errors people repeat are collected in the lighting mistakes that cost the most plants.
Questions readers ask about this
Anubias barteri var. nana, with Java fern close behind. Both run on 15 to 30 PAR, need no injected carbon, and attach to wood or rock rather than rooting, so they survive rescapes. The only common way to kill either is burying the rhizome, which rots it within weeks.
Sometimes, slowly, and only with strong light. Without injection you need close to 50 PAR at the substrate and the patience for six to ten weeks of very little visible spread. Below 40 PAR it climbs toward the surface instead of carpeting and the shaded base detaches.
That is normal and is not a lighting fault. Crypts grown emersed at the nursery shed their entire leaf set when moved to submerged conditions, sometimes within ten days. Leave the roots undisturbed, keep dosing, and new submerged leaves appear from the crown in three to four weeks.
They do, and it is the most common way people ruin Anubias. Above roughly 60 PAR these slow growers cannot produce new leaves faster than green spot algae colonises the old ones. Shade them behind taller planting, or drop the photoperiod by an hour and dose consistently.
