How to make red root floaters (Phyllanthus fluitans) redder?

Introduction

Phyllanthus fluitans is a floating plant from the Americas tropics. It has round leaves that sprout alternately on a short stem, and tufts of short red roots. It is from these red roots that the common name 'Red root floaters' came about. It is a decorative floating plant that propagates very quickly if nutrient rich water and high light levels are available. They are very useful in aquariums for absorbing ammonia and nitrogenous waste compounds. The root system also provides a home for micro-organisms and the leaves provide a much appreciated cover for shy fish. For many aquariums that do not have the conditions of growing submerged plants well, floaters such as Phyllanthus fluitans are a great stand in.

As a floating plant, they are closer to light sources and also able to uptake carbon dioxide from the air - this means that they are not limited by the gas exchange limitations of under water growth. They usually propagate quickly as long as there is sufficient light and nutrients available.

The leaves can be either green or reddish. The main factor that gives rise to red leaves is being deprived of nitrogen. Even plants grown in low lighting will become very red if deprived of nitrogen - the opposite is also true; under high light levels, it can still grow completely green leaves if it has ample access to nitrogen. For aquarists seeking to redden their red root floaters, growing them in tanks where there is low nitrate levels in the water column, and low fertilization levels will quickly redden the leaves.  Under richer dosing and higher nitrogen levels, the leaves will be green and the plant will propagate much quicker.

Red root floaters with reddish leaves grown in water with low nitrogen levels at 189 umols of PAR. This tank is dosed with APT 1, which does not contain Nitrogen.

Red root floaters with greenish leaves grown in water with higher nitrogen levels at 436 umols of PAR (more than double compared with picture above). This tank is dosed with APT Complete which contains Nitrogen.

In densely planted tank dosed with APT Complete. Nitrate limitation due to higher plant density in the tank is turning the new leaves reddish.  

Adding a bunch of Red root floaters from a nitrogen-limited tank to a tank with higher level of nitrogen available in the water column; the old leaves remain reddish but the new leaves produced are completely green. Older leaves have limited plasticity to accommodate new conditions, however, the plant will grow new leaves optimized to make use of the newly available resources. 

Key success factors

• Minimal light levels are necessary, higher light levels give faster propagation rates
• Low nitrate/nitrogen levels in water column turn leaves redder
• Overly turbulent flow can be damaging to the plant, gentle current is preferred
• Dosing nutrient liquid fertilizer is not necessary in most tanks that have other sources of nutrients such as livestock/soil. However, it can greatly accelerate propagation rates
• Be aware that large numbers of floaters can block light from reaching aquatic plants growing in the tank substrate

Propagation & trimming

They grow fast without much input as long as there is sufficient light and a source of nutrients. Excess plants can be picked up and discarded. In tanks where Red root floaters propagate quickly, they can cover the water surface so densely that surface gaseous exchange is impeded and cause oxygen levels in the tank to drop. Floating corrals can be used to limit where they spread to.