if youre afraid of it falling in your tank its best to avoid it plus all those goo and sticky substance, i wouldnt even put it in my tank.
im sure you can use any other small object instead of blu-tack

 

What about using some silicone/plastic tabs that are sticky on one side and designed to go on the bottom of stuff? (IDK what they're called or how to describe them well... you know what I'm talking about? LOL)

 

Like the ones for furniture? I'll look at what they have at the hardware store next time I go.

 

Is there enough room to use 1" rubber balls, cut in half and placed on each corner? I use 'em to prop my t-5 just off the glass. Just make sure they are non toxic. Most dollar stores carry these!
Just a thought

 

LOL!

Furniture glides (if they're intended to slide) or rubber feet (if not, and also available at Radio Shack). You may also find "Grippers" brand high density self adhesive foam pads, which are handy.

It does help to know what things are called when traveling through the Home Labyrinth.