Safe?

You guys think it's safe to have my tank sitting on carpet with a uth attached to the bottom or should I put bigger spacers on the bottom??

I set my tanks on pieces of scrap lumber to keep the UTH off of the carpet or tabletop.

Always err on the side of caution, put more space. You may not start a fire or anything, but if it is not vented well, it could shorten the life of the UTH

Just make sure it's adhered properly to the bottom of the tank. When I raised mine off the carpet I had to use double-stick industrial strength tape and even that didn't help so I had to shove a few things between the tank and the carpet (just in small areas to keep it touching the tank).

If you're using only a thin layer of substrate over the uth, it should not need to ever get very hot to provide an appropriate hot spot on the surface. And it should definitely be regulated to ensure it doesn't overheat! Under those conditions, the uth should not ever get hot enough to damage carpet. But that said, it never hurts to put something underneath it that's unlikely to burn or melt if your dimmer or thermostat should fail.

It won't start a fire, but will affect the uth airflow. I just put 3 1-inch wooden planks under my tank to have some airflow.