Duckweed

Growth and The 'Blackout' Danger:

The Light Blocker. Because it has infinite access to atmospheric CO2 and is closer to your lights than any submerged plant, it will grow with terrifying speed. If left unchecked for even two weeks, it will form a massive, thick, impenetrable, suffocating blanket across the entire surface of the tank. This will plunge your tank into pitch darkness, instantly starving and killing all your expensive, light-demanding planted plants at the bottom. You MUST aggressively scoop out and throw away 80% of the Duckweed with a net every single week.

The Ultimate Nitrate Sponge (The Savior):

Despite the hate, it is incredibly beneficial for heavy bioload tanks (like Goldfish or Turtle tanks). It acts as a massive, aggressive chemical sponge, sucking toxic Ammonia and Nitrates out of the water column faster than a mechanical filter, keeping the water crystal clear and pristine.

The Achilles Heel (Surface Agitation):

The 'Drowning' Weakness. The only thing Duckweed hates is turbulence. If your Hang-On-Back filter produces a massive waterfall that crashes down, constantly pushing the tiny duckweed leaves underwater or splashing the tops of their leaves, they will rot, melt, and die. They strictly demand still, glassy, stagnant surface water to thrive.