I received a great question from Youtube.com friend, Liveoakmerlin
1. Very hardy and tolerate near freezing temps
2. They eat mosquito larvae
3. They are great for the pond ecosystem
4. They do not harm your plants!
5. They are cheap and easy to replace.
I have a couple goldfish right now that are 10 years-old! So, yes. Goldfish and hyacinth go well together. I have goldfish in my personal pond and the hyacinth are growing like mad.
Do not let your water go green. There is no need for that and it will take forever to get the green gunk out. A first-year pond typically has algae problems. Drops from the pet store (located in the pond supply area) will take care of any green algae. Normally, algae will party if the water is stagnant and is in full sun all day long. (Similar to a fish tank in a sunny window.)
Only feed your outside goldfish one time per week. They do just fine with little food, because they also eat any bugs or larvae in the water. If you feed your fish too much they will die. The reflective black water is found with ponds that are very deep. Your primary goal should be clarity. I would suggest water hyacinth, water lettuce, and submerged plants such as anacharis or hornwort. I have all these plants available if you need them. Also, all of these plants provide an excellent breeding environment for fingerlings, baby goldfish.
Thanks so much for writing!