You are not making any mistakes that any other first time fish parents don't make. Look at JLK'S sticky to see size differentiation. Leave the little guys where they are. Set up your larger tank and fill with some of the seasoned water from the current tank. Move the larger fish into the new setup and finish filling with fresh water after a few hours of adjustment. It will mimic a large water change for both tanks. The new tank will still experience a cycle period before those beneficial bacteria introduced with the seasoned tank water take hold and multiply enough to adjust to the proper ammonia processing to nitrate level. Be prepared to do water changes as needed per LIQUID test kit. (Was a pool proffessional) test strips are never truly accurate enough. I still learned that lesson here from the moderators. I never realized how unreliable those test strips were until I compared them to a liquid test kit.
I was determined to have a balanced ecosystem in my tanks. Totally worked with my 2 sight impaired fish. My 55 is a gorgeous heavily planted tank. My Orandas and Telescope in the 120 mowed totally everything, even things that were considered hardy for goldfish (rapid growth rates). I happened upon the best tip ever......Mix artificial plants with real ones....totally a winner! It is even hard for me to tell at a distance whether it is the real or artificial plant. It has totally confused the fish kids. Before, nothing survived. By providing them with a high majority of veggies and natural vegetation, they haven't even disturbed some Wisteria floating at the Surface. I grow Duckweed, Anacharis and other natural vegetation that is intended for food in a dedicated plant tank. This kind of satiates their natural need for plants. The light you have would most likely be great for some lower light dependent pants (I don't know what the spectrum is)? I also will not introduce lead into my water. All new plants after quarantine and then later sterilization with potassium permangenate, are weighted with nonlead fishing weights. They come in a majority of sizes and you can match to the need of sinking weight of the plant. Silly fish try to uproot them...lol
Goldfish in ponds need shade from the sun in the form of Water Lillys and other plants that provide surface coverage.
It is true that your Fancy Goldfish at some time will have trouble competing for food with an ordinary Comet, Shubunkinkin or Koi. A Fancy Goldfish was bred with shape in mind not speed. Their awkward shape is encumbering. That guy would do best in his own tank or a pond.
You are doing a great job and this is definitely a great place to learn. The moderators here are the best
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