[pvaldes]

Take in mind that the common species of mosquitofish is a nasty invader in Europe and this is "advice from 70's". We know better currently. --Don't do it--.

Some mosquitofishes and many killifishes in America are endangered. The common Gambusia will eat every egg of native fishes. If you spread the invasive ones in the ponds of the endangered ones, this can have serious consequences even in USA. Irreversible genetic contamination, parasite spread or local extinctions by predation can occur.

[modeless]

I'm talking about California, not Europe. Mosquitofish are native to North America. Sure, don't spread them in natural rivers or lakes but there's nothing wrong with keeping them in a manmade pond in a backyard in California.

[pvaldes]

The two species of mosquitofish used normally aren't native from California but its main problem is that they are boring as a can. There are much more interesting options:

Like the Coastal Threespine Stickleback. Sticklebacks are the choice fish for small wild ponds in Europe, and is a californian native also.

or Fundulus parvipinnis, the Californian killy fish. Stands saltwater to freshwater. Common near the coast and better than Gambusia (avoid the invasive species Lucania goodei that is similar).

Or a single species of a pupfish like the Amargosa river pupfish or the Salt Creek pupfish, small native tolerating an extreme range of temperatures. Some pupfishes are endangered so each new pond counts. I assume that there are legal exchanges of this species breed on captivity among serious aquarists but check your local laws.

Or the California Roach

If you don't mind natives from California, the fathead minnow, or the American Flagfish are two small US extremely hard species. First stands poor oxygen levels and complicated alcaline water. The second is from Florida and a really cool species, much more interesting to watch than a dumb mosquito fish. They will eat mosquitoes and algae also.

All this species are small fishes (growing less than 10cm normally) so will fit in most garden ponds. Just one species in a small or medium-sized pond. Pupfish species shouldn't be mixed to keep the genetic lines pure. Goldfishes disturb the mud and make the water turn green.