HC refers to Hemianthus callitrichoides 'Cuba', sometimes also called dwarf baby tears. MC refers to Micranthemum 'monte carlo’. In the picture above, MC is grown from the top left, while HC is grown on the bottom right. At the region where they meet (the white dotted line) it can be hard to distinguish the two.
Both are common and popular. And from a distance, look very similar. So similar, in fact, that many shops sell them almost interchangeably. “HC is just a smaller MC” “Oh either would be fine” Nothing can be further from the truth.
Above: MC is a great foreground carpet. From a distance, it is hard to tell the difference between MC and HC.
They do look very similar. HC has the smallest oval leaves among aquatic carpets, the size of duck-weed. MC is quite a bit larger if grown under robust growth parameters. At maximum size, MC leaves are around 3 to 4 times larger than HC. However, in lean and over-crowded conditions, MC can be very similar in size to HC. But that is where the similarity ends.
Night & Day
Above: Monte Carlo is very hardy. It can grow on wood or rock even without substrate, and can even grow in tanks without CO2 injection if given sufficient light and time. The trick in growing them well without substrate is to provide good nutrition in the water. In the picture above, we use APT Complete.
In comparison, HC requires a high level of CO2 to grow well, and requires soil substrate to grow well. While MC takes overcrowding quite well, the bottom layers of HC deteriorates very quickly once overgrown.
In short, while MC is a really hardy carpet, HC is, by comparison, extremely picky.
The two may look similar, but are miles apart in the care and conditions required to grow them well over time.
Good to have
Of course there are outliers, and the internet is a hotbed of survivorship-bias "oh look- I grew HC without CO2" but the real test is consistency over time, and the overall rate of success. It is far more helpful to see MC/ HC as vastly different, than to see them as 'same-same'.
MC and HC Maintenance
Both MC and HC require pruning (in a CO2 injected tank with strong light, every 2 weeks if not 3) to remain in good form. With MC you can get away with pruning less frequently, while HC requires a much (repeat) higher level of discipline and regularity.
What happens if we do not prune? The top layers shade the lower layers which start to deteriorate, as shown in the picture above. With MC the process is more gradual. With HC this happens far more quickly. You cannot substitute regular pruning with better or more nutrition / fertilisation.
As a general rule, trim sooner and earlier, rather than later. It is far better to trim aggressively, rather than leniently.
Once the bottom layer deteriorates, you need to replant healthy parts of the top layer.