PVC inside walls sludge build up?

[Delphinus]

Someone ought to market a pipeline pig for aquaria scale pipes ..

...

I wonder the same thing. I would imagine over a large enough time period it could become an issue. My guess though is that aquaria get changed around often enough that the pipes would just be be replaced anyhow before it got to be an actual issue. Just a guess, though. I don't know of too many people who have had the same tank, in the same spot, using the exact same hardware, for more than, say, 2 or 3 years. There's always something to upgrade or change around or somehow make "better".