New to me 50 gallon

[Bobo8]

Just recently got a 50 from a friend. It is a tropical tank with a few live plants. It wasn't in the best condition so I want to slowly get it back to good. Currently there is a black moor goldfish, 4 rummynose tetras a cherry barb, a female bristlenose pleco, one green Cory? And a synodontis petricola and a khuli loach. What I want to do is move the goldfish into the 36 gallon tank I have my own few tropical fish in which I want to move to the big tank. Then convert it to a goldfish tank.
A few questions I have. Both tanks are fully cycled but the 50 has extremely poor water quality and the fish have been in it for quite sometime.

How much water can I change and how fast?
Here are the water parameters.
Ammonia 0
Nitrite 0
Nitrate probably 160 plus I'll post a pic of the test.

So far I have only done a 5 gallon water change and then added in 10 gallons. Also scraped the algae off the glass.
I dint want to shock the fishies.

Also any advice on goldfish tanks would be awesome. I've never had one. Only tropical fish. I have definitely done research but advice is always awesome.

Is a 36 okay for 2 goldfish if I keep up with water changes and testing? Or is the bioload too heavy?

Here are a few pics. Thanks in advance!!!

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[Mebbid]

I would start with 10% changes daily. Do that for a week and bump it up to 20% every other day. Do that for a week and you should be good for 50% changes after that.

Personally, if it were me i would pull the fish all out and put them into a rubbermaid tote with an air pump. Empty the tank then fill it with fresh water. Then id drip acclimate them over 12 hours or so and call it good. Id probably shoot for dripping 10% of their holding container per hour removing the excess.

Check ph and temp of both. If they match at the end then they should be good to add in the tank.