TIP120 Transistor +arduino

i have a arduino mega2560 and a instalation on my aquarium 8 leds (10watts 1 amp each one) I have connected two LEDs for each TIP120 and one resistor 1k for each one the electric current is not constant when i try to dimmer my leds,the light is unstable the resistor is a good idea for my tip120 o i need change to 2.2k resistor? the time try to control my leds is one hour 0 to 255 . or i need do a bridge diode retifier to control the variable current?