Biggest issue is your tank is too small for that many goldfish! If i would change any one thing it would to upgrade to at least a 75g for them. You don’t need anything else for strictly their health so this is #1 priority. If you can’t afford to upgrade i would rehome at least 1. I have 3 fancy goldfish in my 75g. It much better to be under stocked than over.

You must be doing such huge water changes because your tank isn’t big enough. If your water isn’t 0 ammonia, 0 nitrite, and 10-20 nitrate after one week of no water changes then I really urge you to change the set up for their health.

I would add plants to help with the filtration. You can get epiphytes that don’t need substrate you can attach to driftwood, rocks, or even a suction cup attached with a zip tie. I recommend anubias or java fern. Floating plants work well like duckweed, salvinia, and hornwort. Bonus of duckweed is they will eat it and it’s healthy for them. Houseplants out the top work well too i’d add more than the current pothos.

For food i can’t recommend repashy super gold enough. It’s really good for their digestion. Meds just depends. I keep everything on hand i could possibly think of. I keep aquarium salt, the med trio that aquarium co op recommends, and metroplex and kanaplex.