Cold-water 10-gallon aquarium stocking ideas? Goldfish are bad ideas?

[Wayfarin]

30 gallon seems too small for a red eared slider. I kept one in a 55 gal for years and I ended up having it to give it away because that was too small.

It is. But she's certainly not so big that she doesn't fit in a 55. In fact, she's 12 years old now and still fits in the 30-gallon, but she needs more space to move freely.

 

[Wayfarin]

I'm going to post a separate thread on gobies and pistol shrimps.

 

[Abdulkarim Elnaas]

From what I know, fish (like goldfish) that stop growing in a small environment suffer because their organs never stop growing and they eventually die from organ failure. A 10 gallon really isn't good for either goldfish or dojo loaches because, although their ammonia tolerance can be high, both species (but especially goldfish) produce so much ammonia that they far exceed their durability quicker than a fragile fish that doesn't produce much ammonia would. And the truth is dojo loaches are durable but fancy goldfish are actually kinda fragile in my experience. Fancy goldfish actually don't thrive in coldwater environments and comet tail goldfish get waaaaay too big for even a 20 gallon to house them for long. That said, what exactly do you mean by coldwater? I reccomend you watch the YouTube video by aquarium coop on heaters:

This video changed my perspective on coldwater vs tropical fish. You might find that you have more options than you think. My reccomendation? Shrimp are fantastic. They are extra low maintenance once they become established and when they breed you can also sell them back to your local petstore. They also take care of java moss which you can also grow in a 10 gallon and sell to your petstore. I also think a single male betta is another great option. The colors that are available these days blow my mind. Nowadays even common stores like PetSmart sell fancy breeds like Blue Mustards which are absolutely breathtaking. Also shoutout to fathead minnows (I'd just get one male and one female in a 10 gallon) and white cloud mountain minnows (gorgeous, bulletproof, coldwater fish).

 

[Wayfarin]

From what I know, fish (like goldfish) that stop growing in a small environment suffer because their organs never stop growing and they eventually die from organ failure. A 10 gallon really isn't good for either goldfish or dojo loaches because, although their ammonia tolerance can be high, both species (but especially goldfish) produce so much ammonia that they far exceed their durability quicker than a fragile fish that doesn't produce much ammonia would. And the truth is dojo loaches are durable but fancy goldfish are actually kinda fragile in my experience. Fancy goldfish actually don't thrive in coldwater environments and comet tail goldfish get waaaaay too big for even a 20 gallon to house them for long. That said, what exactly do you mean by coldwater? I reccomend you watch the YouTube video by aquarium coop on heaters:

This video changed my perspective on coldwater vs tropical fish. You might find that you have more options than you think. My reccomendation? Shrimp are fantastic. They are extra low maintenance once they become established and when they breed you can also sell them back to your local petstore. They also take care of java moss which you can also grow in a 10 gallon and sell to your petstore. I also think a single male betta is another great option. The colors that are available these days blow my mind. Nowadays even common stores like PetSmart sell fancy breeds like Blue Mustards which are absolutely breathtaking. Also shoutout to fathead minnows (I'd just get one male and one female in a 10 gallon) and white cloud mountain minnows (gorgeous, bulletproof, coldwater fish).

Yeah, I've given up on keeping goldfish and dojo loaches for the most part, except for any feeder fish that might avoid getting eaten by our red-eared slider in her large tank. I agree with your comment about fairly hardy fish with a lower bioload being easier to keep than very hardy fish with a high bioload.
However, I don't actually believe that growth stunting kills goldfish, since many tiny goldfish have lived a very long time, sometimes over 40 years.

I love shrimp, especially cherry shrimp. However, I'm not sure if they really count as cold-water. I also like trumpet snails, but they apparently need heaters, too.

I would not consider a Siamese fighting fish a "cold-water" fish where I live, in the Northwoods. Maybe in Florida.

I love fathead minnows, but I'm not sure if I'd want to keep them as pets by themselves, since any feeder fish that our turtle decides not to eat can become our "pet" minnows.

I rather like least killifish, pygmy killifish, bluefin killifish, pygmy sunfish, and swamp darters. They are cold-water if I recall correctly?
Unfortunately, though, most sellers don't have them. Although I've heard that least and bluefin killifish sometimes "hitchhike" in shipments of ghost shrimp?