I need help here. When building a concrete pond would it be better to leave the walls & floor concrete plaster with sealant or would it be better to fiberglass it?

it depends, what is your budget, plaster is much cheaper.........

I get the fiberglassing at cost so it's not that much more expensive.

Let me put it this way:
I've got an outbuilding which I'm looking at converting into an indoor pond. The walls are painted and the floor is carpeted. Then there's a bathroom with tiles etc.

Once I've scraped all the paint off, got rid of the carpet & removed everything that needs to be removed and I'm left with an empty shell. What would be best for the fish & water, plastering or fiberglass?