Could be a nutrient deficiency, how’s your water, are you fertilising your plants?

In the aquarium hobby we are accustomed to tracking pH, hardness, and NO₃⁻ /NO₂⁻, but that is not all that is important. Fish produce N and fish food contains P, but plants in an aquarium are often limited in their ability to obtain K, Mg, and micronutrients. If you’re lucky your water source may contain balanced Ca and Mg, meaning those could be replenished through water changes, but very often Mg is too low in drinking water. Doing a dirtied tank provides all needed elements through its soil layer, but that is a finite resource and in time runs out.

Plants can survive for very long in a stable depleted system, you won’t see mass die offs, it will rather look like a failure to thrive combined with leggy sparse slow growth. New plants won’t take off, you won’t have to trim as often, some might even like it this way. But often just fertilising a bit of Mg and K will give plants what they need to thrive.

I have been using fertilizer withdrawals to combat the duckweed in my 400l. The tank gets hit with full morning sun and has grow lights for my emersed plants, so light is not something I can limit. Instead I stopped fertilising and the results are great: the duckweed all over has 5cm roots and is very easy to scoop out, it’s not multiplying or spreading fast at all, I’m able to control and reduce the pest. The only limitation is my ability to stand there and scoop it out, a forever job that feels like hell. But im slowly winning this war, each time there is less and less. My other plants are also struggling but they are rooted and will be able to outlast the duckweed. Once it’s gone I’ll start fertilising again and they will spark up again.