Echinodorus: Sword Plants
The genus Echinodorus, the Sword Plants from South America, are among the most used aquarium plants and some are among the easiest to care for. Most will outgrow a 10-gallon tank. Not all are from the Amazon basin, though it's the center of their diversity; three grow far to the north through Mexico and into US territory as far as California and the Ohio River valley, where the Spadeleaf Swordplant, Echinodorus cordifolius, survives wherever the water doesn't freeze down to the substrate, and the dwarf E. tenellus makes a dense turf. Other habitats extend southwards from the Amazon basin: the big Echinodorus uruguayensis is from Uruguay, naturally.
Kristel Kasselmann points out that Echinodorus species cannot survive the long, deep annual flooding undergone in a varzea in the central Amazon basin: instead, they thrive along the fringes of distant catchments of small tributaries, growing emerse during the dry season. Most are stream-bank or marsh plants by nature, and some make themselves unhandy in the aquarium by insisting on throwing up floating or aerial leaves: very nice in an open-top tank, where you can really enjoy the long flowering shoots with flowers at the nodes, which they'll stretch above the water surface, instead of pressing them below the water surface, as I have to do. Underwater the flowers don't open, but the shoots produce adventitious plants at the flowering nodes: when the young plants have grown a good set of roots, you can gently twist them free and plant them right up front for a while, until they get too big for that position.
Many of the best echinodorus for aquariums are hybrids. Echinodorus leaves are variable: the submerse leaves are soft in texture and more translucent, sometimes lighter green, or perhaps tending to olive-green or even reddish when they're new. The emerse leaves are leathery and opaque, protected by a waxy cuticle suited for their dry environment. Plants you purchase are likely to have been grown in low water levels, with rapidly-maturing emerse foliage; those leaves will die off in the aquarium, but new, genuinely submerse leaves quickly take their places. The two kinds of leaves may differ so much in shape that species have been unnecessarily multiplied by unwary botanists, to accommodate each variation. Conversely, Christel Kasselmann notices in her Aquarium Plants, dried herbarium specimens and details of flower and seed may unify as one species plant populations that differ a great deal when they're being grown in aquaria. Since she wrote, the taxonomy of Echinodorus has been revised and argued over during the first decade of the 21st century; the discussions are now based on DNA rather than the variable and misleading morphology, giving some 28 natural species.
Echinodorus that you may see being grown in discus aquariums are often getting by at the very top of their range of temperature tolerance, at 86o; at about 88o leaves and even crowns may seem to have melted irretrievably: if you refrain from handling or moving the plant, the rosette may regenerate with cooler conditions, or you may have several small surviving offshoots.
Characteristic long narrow leaves have given Sword Plants their common name, though my small tanks and modest light result in swords that are more like butter knives. If you need a bold specimen with wide, ovoid or heart-shaped leaves instead, look for E. cordifolius, the Spade-leaf Echinodorus. It is best growing as a single specimen, which will need 18" of water depth. Strong light in a daily period of 10 or 11 hours will discourage it from throwing out emerse leaves and flowering, for this is a long-day plant, whose flowering is actually triggered by short "nights" rather than long "days". Timers on your lights are essential equipment. Some enrichment in the lower level of the substrate will encourage it, if you need it to be larger - Philip Brown featured it as the "Mud King" in Tropical Fish Hobbyist - and in an old tank with too much phosphate already accumulated in the substrate, a strong-growing E. cordifolius may fill the root-zone, take up the excess nutrients and give the system a new lease on life. I keep E. cordifolius under control in the rather cramped conditions of a 16-gallon tall tank by periodically removing the oldest, outside leaves, which are often hudden anyway under the newer ones that arise from the center of the rosette.
Link. Curt Quester's page Echinodorus on-line is a mine of information that includes ecological descriptions of Echinodorus growing in situ, as well as detailed botanical descriptions to help you identify which species you are growing.