Water Lettuce

The rosettes are only half the plant. Below each one hangs a mass of feathery roots that reaches 30 cm or more in a mature specimen, and that root curtain is what makes water lettuce worth keeping: it is both a nursery for fry and shrimp and a large surface area for stripping ammonia, nitrate and phosphate out of the water column.

Above the surface it forms soft, ribbed, velvety rosettes that do genuinely resemble small lettuces. Holding that compact shape takes strong light. Under weak light the rosettes stretch, pale toward yellow-green, and lose the form entirely, which is the usual reason it disappoints. The other common failure is condensation and splash from a tight lid, since water sitting on the leaves rots them. It suits open-topped tanks far better than covered ones.

Warmth suits it, at 22 to 30 °C, with growth quickest at the upper end. It takes pH 5 to 8 and a wide hardness range. Like other floating plants it takes carbon from the air, so injected CO2 is irrelevant to it. It spreads by stolons that carry daughter rosettes, separable once they are a few centimetres across. Growth outdoors in warm climates is aggressive enough that the species is treated as invasive in parts of the world, so it should never be put into local waterways.