I have just bought a 10w 900mha led with 9 - 10v forward voltage, I am planning to use this to light a small fish thank, what would be the best way to power this bearing in mind it may need dimming down as it could be too bright, any help woulf be great guys thanks in advance

 

I recommend shopping for an LED driver meant for the LEDs you want to use. Making one yourself will be a lot of work and cost more. If you enjoy doing it anyway, you should at least reverse engineer the commercial solution.

The driver you want will be able to handle at least 1A (1000mA) and will employ pulse width modulation (PWM) dimming.

 

if you are making it yourself then have a look at the LM317 voltage regulator whit a big enuf heatsink, plenty of circuits to find on google

 

If you are planning to have plants inside the tank then you might want to reconsider your lighting system as the LED's might not give an optimum light spectrum for them.
Also you might have algae growing in your tank . Read up on it

 

You could use a current limiter to drive the Led with an LM317, on your 12v supply, but you need to know if the wave length is correct for the fish , any datasheet ?

for 900mA use a 10 ohms pot. current is 1,25V/resistor ohms

circuit

 

I suppose 10W is not a lot, but one reason to use a commercial driver is that it will use an efficient switch-mode power supply to power the LED. Any solution based on the LM317 or other "linear" devices can waste as much power as heat as the LED is using.

 

but you need to know if the wave length is correct for the fish , any datasheet ?

Fish don't care what the wavelength is..
Now if its saltwater corals or planted freshwater tank thats a different story.

 

Just get a Meanwell LPF-16D-15 and a 75k potentiometer for its dimming input. DONE


BUT you MUST attach that LED to a decent size heatsink or it will overheat and go bye bye.