ahh, the rope light rumor
-yes- one could use rope light, but then if going that route, why not just use a pair of those blue party-bulbs they sell for $1.99
the problem with this is the color purity of the light. rope light and party-bulbs produce light of about 2700-3000K (via tungenston filament). So you take this predominately red/yellow light, and put a cheap blue filter over it.
The result is very little usable blue light, along with plenty of red and green, not to mention a considerable amount of heat.
or neon moonlights produce light using a more advanced technology, and produce it a lot more efficiently and precisely.
Go to an auto parts store and look for neon accent lights ... this is what I currently use for my 30 gallon and they work great ... I have 2 11" bulbs taped inside my light fixture, and they light the tank almost too well.