Hi, hope this helps

Qn1. May I suppose, for an n-feet DeLighting fixture, the distance between the end-caps will always be "n++" feet apart? i.e. a 2-foot DeLighting fixture will have the end-caps , perhaps 2.1 feet apart?

The encaps will follow the bulb length. Perhaps extending out a cm or so?

Qn2. Any side-effects from resting the end-caps onto the the tank edge (even with a pipe or 2 propping)? i.e. constant heat transferred from the end-caps onto the tank glass will cause the tank glass to crack?

I doubt so. Heat is felt from the tubes, hence radiating to the surface of the water. Not the glass. If you have sufficient height it would be better. I'm facing heat issues.

Qn3. Naz, I couldn't see clearly from the photos, but your tank "edge" is actually a narrow strip of glass (perpendicular to your tank's vertical side) right? i.e. it ain't just a thin vertical edge of glass right?

Yups, its resting on the tank brace, actually it exceed abit because the metal casing has about an inch or so extended side ways to accommodate holes to install cables. Therefore it hits against the cabinet.

Alternatively perhaps you can mount it via cables from the ceiling or a brace to the wall that supports the light. Perhaps like a shelve. Kill 2 Birds with one stone, have a shelve and it holds the light fixture. Jus an idea..