Duckweed

Duckweed is one of the smallest flowering plants on earth, and it's found on every continent except Antarctica, colonizing ponds, ditches, and slow-moving water with zero effort. Each frond is a tiny flat oval no bigger than a few millimeters, and they reproduce by budding so aggressively that a small handful can carpet an entire tank surface within weeks.

That rapid growth is exactly what makes it useful. It pulls nitrates and phosphates out of the water column faster than almost any other plant, making it a genuinely effective biological filter for overstocked or heavily fed tanks. It also gives fry and surface-dwelling fish a sense of cover they really seem to appreciate.

Low light works fine, CO2 is unnecessary, and it'll grow in almost any water parameters a tropical fish can tolerate. The catch is containment. Once it's in, stray fronds hitchhike on nets, equipment, and hands, so keeping it out of tanks you don't want it in requires real discipline.

Goldfish, cichlids, and most herbivores will eat it readily. If you're thinking about adding it, look at setups using it as a nutrient export strategy alongside regular skimming.