Hi Robert I am in the process of putting togather a 108 G reef tank and looking torun 1/4" tubing between my living room and garage, 3 lines. 2 lines will be to drain and refill salt water in my main tank. The plumber wants to run copper under the house. Would 3 plastic tubes inside a PVC pipe be better
Yes, way better.
Tip: Use Electrical PVC conduit (grey stuff), not Plumbing PVC for the outer sleeve tubing.
The conduit has UV protection and has nice wide bends for elbows. It is not pressure rated, but you do not care about that.
The elbows in particular are an issue, since normal plumbing elbows can kink tubing inside, or make it really hard to run lines.