I have a some experience with aquariums but I'm new to ponds so I would like
to know if I'm planning everything right.
I want to start a pond in an already made concrete pond, it's dimensions are
125cm x 125cm x 35cm, about 550liters or 140 galons.

The first problem I see is that it's not too deep, I would have done it much
deeper but modifying it is not an option, is this a big problem?, I'm
planning to have some goldfish in it.

The second thing I would like to know is how much goldfish I can have there,
I calculated 7-8 fish at most (specially when they grow), but I read that
10 galons per fish is ok, is this accurate?, will all those fish live ok
there?

Third question is, what filtration media can I use?, I was thinking bio
balls, but they are very expensive in my country. Any diy and cheap
solution?

Fourth and last, I have a powerhead that can drive ~1000liters/hour is this
ok for this pond?

Thank you very much.

Francisco