Who Gives A Fish Forums

Duckweed is taking over my 20 gallon tank, it's covering the surface completely. I've got a sponge filter, moderate lighting, and do weekly water changes. How do y'all keep it in check without nuking the whole tank?

Manual removal is the only way. Scoop it out daily with a fine net. Duckweed reproduces fast, so you gotta stay on top of it. I run a high flow canister filter to keep the surface agitated, slows it down a bit.

I've found that reducing light exposure helps. Duckweed thrives under bright conditions. Try shortening your photoperiod to 6 hours and see if that slows the spread.

Yeah, duckweed's a pain. I let mine grow wild in my shrimp tank, but in my main display, I gotta scoop it every other day. Goldfish love munching on it though, so maybe get a couple if your setup allows.

Introduce floating plants that outcompete duckweed, like Water Lettuce or Frogbit. Their larger leaves block light and roots absorb nutrients faster. Works in my 40 gallon with CO2 injection.

Thanks for the tips, folks. Gonna try reducing light first and see if that helps before adding more plants or fish.

gunsmoke wrote: ↑Mon Jun 23, 2025 5:19 am Manual removal is the only way. Scoop it out daily with a fine net. Duckweed reproduces fast, so you gotta stay on top of it. I run a high flow canister filter to keep the surface agitated, slows it down a bit.

I agree, but it's such a chore. I've got a turkey baster I use to suck up clumps, way easier than a net.

Duckweed's a beast, man. I gave up and let it run wild in my betta tank. The little dude loves hiding under it, and it keeps nitrates low. Win win, I guess.

If you're gonna manually remove it, do it before water changes. Less chance of it spreading when you're already disturbing the water. Also, check your ferts, duckweed goes nuts with excess nutrients.

Surface agitation is key. I pointed my HOB filter output slightly upward to create more movement. Duckweed hates it, but my other floaters are fine. Just gotta find the right balance.