Welcome !
You may want to consider getting a sturdy bin or pond for your Goldfish, Koi and Pleco since all 3 types need very large tanks.
For now ?
Cheap indoor ponds can be made from a sturdy plastic bin or water trough. You can buy or DIY a large filter. YouTube has a ton of DIY pond vids. You can make a filter with a new/clean 5g bucket.
A single Common GF prob needs 30-40g to grow and stay healthy. They can hit 8-12" and live over 20yrs with good care.
Koi hit 18-24" do best in ponds and can live 50-100yrs with good care.
Common Plecos hit 12" and need 125g tanks or bigger to thrive. They can live 25 yrs.
https://youtu.be/umUOh5eXu3s
Here is one example
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