im thinking of adding another 2 lights + reflectors into the hood of my rio 240, could this give off too much heat and make the temp fluctuate in the tank when the lights turn on and off? if so any ideas?
Unless they're incandescent or metal halide, fluorescent shouldn't change your temperature too durastically, how big is the tank? As long as you have some opening in the canopy for heat escape, I'd go for it
You will be fine. The Juwel Rio 240 is designed in mind to add another twin light bar with no consequence to water temp. I have squeezed in additional tubes on my Rio 125 to good effect.
Bear in mind that if you double your lighting then you may need to increase other nutrients i.e. CO2, ferts. to keep everything balanced otherwise you may grow algae more effectively than plants. HTH