decomp does add nutrients, but over a LONG period of time. The bug isn't just dissolving nutrients into the water. Fungi and bacteria are decomposers, they will consume chemicals from the dead things and produce waste which is used as nutrients for other things. Plant decomp can happen much faster and provide nutrients for other plants much more quickly than bug or animal decomp, but even then.. just having a dead plant in the water is unlikely to provide such an immediate result.