Advice on using copper/brass pipe work in filter

Does anybody have anything to say on the subject of copper/brass fittings
when building your own bio/mechanical filter system?

I have just built a small filter system for my new pond (2600 gal), and
have studiously avoided using any metal fittings, but I am not very happy
with the impedance that the plastic fittings introduce into the water
flow. In addition the range of fittings available in plastic is not as
great as that available in brass.

Everything I have read here to date seems to indicate that I should avoid
using metal in fishponds.

Any advice would be most welcome.
ANO