Goldfish are one of the most difficult fish to kill. They revert back to their original form ie Carp, if left to breed on their own. So that might be the brown fish. But I know people who get some sort of native fish that eat mozzies for their fish ponds and they're brown too - could be them.
There are supposed to be certain pond dwellers that reduce the amount of algae like aquatic snails. I've heard of people using garden snails to get rid of stuff that grows in shower alcoves - though I'm not sure which I'd prefer between snails or mildew stuff.
And certain plants help reduce algae - if you can reduce the amount of light getting in, that helps, and what they said about the food. And it would be cool to have yabbies and other things - if the gold fish don't eat them.
And you will need some mesh or Kookaburras, shags and other fish eaters will get your fish - especially if you clean the pond up so you can see them.