I think that you are giving your dahlia
plant too much water if you are watering it daily. Hopefully your
plant is still alive.
I would stop watering for a few days; feel the soil in the pot. If it feels moist, don't water. Yes they like full sun. Okay there. Is the
plant indoors? If yes, dahlias are more of an outdoor
plant. However, if you live where winters are real cold--hard frosts, lots of snow, or where winters are wet with lots of rain, the tubers should be stored in a crate or box covering the tuber with bark dust, or vermiculite and placed in a cool room where the temperature stays around 45 to 50 degrees.
As for watering dahlias, they like to be watered real thoroughly about once a week. Check the soil for moisture and hold off watering again until the
plant slightly drys out. Remember: Dahlias are an outdoor
plant; they are not generally grown indoors except in
greenhouses.
Hope this was of some help
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