I have a similar setup for my Australian Spotted pythons. 20L with undertank heat and aspen substrate. The only difference is I am using a 12" piece of the 11" wide heat tape (from The Bean Farm). I found that this setup was not very efficient so I had to adjust it somewhat.
What I did was cut a piece of plywood the same size as the tank to act as a base (because they are on an iron stand)then I placed a piece of 3/4" foam board between the wooden base and the tank (up in the recess under the tank)this holds the heat tape right against the bottom of the tank and directs the heat upward. If you have your tank on a shelf you would just need to get some foam board.
I found that this made a HUGE difference. I control them with a rheostat and I was able to turn it down to about 40% and easily maintain 88-90 degree temps. It provides a nice heat gradient and works perfectly for my Spotteds. However, Spotted pythons like it a little bit cooler than Balls so you may still need to play with your setup. Depending on where you live, the ambient temp might be a little cool because with an aquarium all of your heat goes right out the top. You may need to cover part or all of the top with a towel or something like that.
Hope this helps you out.

John Yezbak

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