http://www.homedepot.ca/product/cred...r-white/935870
^ That's the one I got. It's not adjustable.

Well then, the instructions for that one say the CFL or LED lamp used "must be marked as dimmable", so you need to confirm if your PAR38 LED is in fact dimmable or not. Chances are, it is not. Have a closer look at it, maybe get a part number & research it.

They are definitely not all dimmable.

Some are made much, much cheaper than some others.

Ive used cheap ones (30-40 a bulb) and expensive ones (100 per bulb) and have been very happy with all of them.

You get better spectrum and build quality in the more expensive ones...usually they are made with the hobby in mind.

The cheap ones seem to be ones that work for the hobby, rather than having actually been designed for it.

On a nano tank, I cant imagine anyone not being happy with these for the money spent.

Are the halos dimmable?