Here’s a rather ingenious hack where the owner of this fish tank has replaced his florescent lights with LEDs. On top of that, he has used aluminum casing that transfers heat generated by its LED system into the water directly. (You can see aluminum bars extended into the water.)
First off, I don’t think the LEDs themselves generate enough heat to actually heat the water much because LEDs in general are low-heat devices. But it seems like the owner of this fish tank has used special LEDs plus the voltage regulator in the LED lighting circuit probably generates a ton of heat, enough to warm the water a bit.
Overall, this hack seems to work well as proven by the author’s power meter, saving about 35Watts. Nicely done.