50, 75, or 100G pond for common goldfish?

[Supermassive]

I have 3 common goldfish. They're in a 29 gallon and I've been struggling with nitrates and they are around 5-6 inches so I want to give them more space. What size "pond" would you recommend for them? I would like it as small as possible without potentially stunting their size or having nitrates build up too quickly. Currently I'm thinking I will just have one or two large sponge filters. I don't plan on putting any other fish in there.

I don't the budget for an actual pond so I'm looking at getting a livestock stock tank. They're around $100-$200 Canadian for 50-100 gallons. Maybe there is a better option I don't know about but that's the best thing I've been able to find.

Thanks for any advice.

[JettsPapa]

I'd recommend getting the largest one you have room for and can afford.  Common goldfish can get large.

[mynameisnobody]

As large as you can, tractor supply sells stock tanks that are 110 and 300 gallon. 110 is adequate for your trio. 
Edit: I’m a dumb dumb, just realized you’re in Canada. 

Edited January 9 by mynameisnobody

[Stef]

Agree with going big. I too have 3 big comets and a young koi in my 100 gal stock tank. Had them in there over summer. Brought them inside so I wouldn’t worry about them over a long cold Chicago winter. They’re in a 40 gallon breeder with two large sponge filters and a canister filter and I have to do weekly water changes. I can’t wait for spring to open up the pond again. 
 

I’ve added some lucky bamboo plants and a pothos to help with nitrates, but the big weekly water changes are my future for the next several months. 

[Tony s]

It all depends on how you want to interact with them. If you want them where you can see them, get a regular tank, but as large as you can. Both price wise and weight wise (depending on the flooring). Then you can dyi yourself a tank stand for around a hundred dollars or less. The heavier the tank, the more bracing you’ll need. Bare bottom tank would be fine if needed. livestock tanks are great. But the visibility is not great.  
 

To help with nitrates, you could use houseplants. There is a product called a pothocarry that is used to hold onto the stems of the pothos plant while the roots are suspended into the tank, using up the nitrogen. Roots are much less edible than the rest. You could use whatever plant you desire. Even veggies, which you could feed to you or back to the fish. I would makes sure the green plant parts stay out of the water though. Depending on what you choose, there may some defensive chemicals in the above ground parts. Usually not in the roots. 

[RachelElizabeth]

I would say go as big as you can afford. Those goldfish will grow FAST and the nitrates are for sure hard to keep up with. I had to help my sister-in-law with a temporary over-the-winter set up for her red eared slider and a bunch of tiny goldfish that ended up in the outdoor pond somehow (eggs attached to live plants, maybe?). Anyway, we did a 300 gallon livestock tank and even with a huge DIY cannister-style filter plus two large sponge filters and frequent water changes, it got gunky fast and the nitrates got really high. Granted we had the turtle and more than 3 goldfish, but I just feel like the bigger the better when it comes to housing goldfish indoors.