Effect of three herbicides namely 2,4-D, metsulfuron-methyl and glyphosate was evaluated on fish mortality and water quality in relation to control of aquatic form of alligator weed (Alternanthera philoxeroides; Family, Amaranthaceae). All herbicides caused fish mortality and affected water quality after application, but it was highest in 2,4-D treated tanks followed by glyphosate and metsulfuron-methyl. Herbicide did not cause fish mortality at 1 DAA (days after application) but it caused at 7 DAA and increased corresponding to increase in concentration and days. Fish mortality was recorded lowest in herbicides treated tanks that were having only water but no weeds. Significantly higher fish mortality occurred in 2,4-D treated tanks having weeds. This reflected that fish mortality was more due to decaying of weeds, which decreased dissolved oxygen drastically in the water tanks. Herbicides did not affect fish development because growth and weight of fish was highest in water tanks treated with herbicides having no weeds. All the herbicides significantly decreased pH in treated tanks than control at 0 and 1 DAA, however, it was resumed towards normalisation in due course. The decrease in pH was least in the tanks having weeds and treated with metsulfuron-methyl followed by glyphosate and 2,4-D. Further, decrease in pH was less in water tanks having no weeds than having weeds. All the herbicides significantly decreased the dissolved oxygen (DO) at 0 day in water tanks with and without weeds except metsulfuron-methyl in the tanks having no weeds. Decrease in DO was more prominent in 2,4-D treated tanks followed by glyphosate and metsulfuron-methyl. Dissolved oxygen was least affected in tanks having no weeds.