What other kind of fish, (except Koi) can I add to an existing goldfish pond that are unlikely to introduce diseases. Any special process on how to introduce or what to treat new fish with, to reduce chance of infection.
I called the mosquito abatement folks and they dropped off about a dozen mosquito fish for free to my garden pond that already had comets. I think they have all adapted well. One thing they told me is that mosquito fish will kill small goldfish so don't add them of your fish are small. My comets are about 8 inches and the mosquito fish are about 2 inches so no big deal. Mosquito fish also bread like crazy and can easily take over a pond, but luckily we are in a cool enough climate they wont survive the winters so again not a big deal. They will also eat your comet eggs faster than the comets so not a good chance of babies surviving.
I may not add them next year, who knows but they are neat to watch and I like having the variety.