If you have high power LED chips, like 1W 3W CREE chips, and so on,

they have good lifetime and high brightness, it is very suitable to make fish tank or aquarium light, but it can not be controlled by timer programmable controller , like TC420 & TC421.

TC420 and TC421 work at DC12-24V, but single LED chip always work under 5V, so you can connect them in series with resistance like below, to make whole unit (LED chain) to work at DC12V or DC24V:

the questions is how many LED chips and how much resistance should I use?

first you should make sure :

voltage of your power supply,

work voltage of your LED,

work current of your LED,

For example:

Power supply = DC12V

Work voltage of LED = 3.3V

Work current of LED = 700mA

first we calc how many LED max. can be connected in series:

Power suppy voltage / LED voltage = 12/3.3 = 3+, so we can connect 3 LED in series max.

then we should find a suitable resistor to limited the current, make the current under 700mA, protect the LEDs,

here we find a suitable solution with 3W cree LED, please check.

you can also check the following solution with constant current dimmable power supply on website: TC420.net:

https://tc420.net/connecting-high-power-LEDs-to-the-TC420.php