The short answer
Floating plants usually fail for one of three reasons: water splashing their leaves (they rot from the top), too little light reaching the surface, or nutrient starvation. They also hate being trapped against filter flow. Calm the surface, give them decent light and a little fertiliser, and species like frogbit and salvinia grow almost too well.
Keep their leaves dry
This is the big one. Floating plants take in air and light from above the water, so a wet leaf is a dying leaf. Constant filter splash, a spray bar aimed at the surface, or condensation dripping from a tight lid will melt them from the middle out. Reduce surface agitation, angle the outflow down, and if you use a lid leave a little airflow. See surface film for related surface issues.
Light and nutrients
Floating plants sit closest to the light, so weak lighting is rarely the cause on its own — but a dim kit light or a tank in a dark spot will still starve them. More common is a nutrient shortage: pale, yellowing fronds mean they’ve run out of food, often iron. A weekly all-in-one fertiliser fixes it.
The easy floaters
If yours keep dying, switch to forgiving species and fix the surface: Amazon frogbit, salvinia, water lettuce, red root floaters and duckweed. They shade the tank (great against algae) and mop up nutrients — just don’t let them cover the whole surface and starve your other plants of light.