Hey and welcome to the forums!
I agree with what's already been said, 30 litres is far too small.
What are your ammonia, nitrite, nitrate readings?
Ammonia and Nitrite should be 0ppm, with Nitrates below 40ppm. If you ever see ammonia or nitrite above 0ppm, do a large water change. Be careful as you might have a spike, I had a nitrite spike in my goldfish's old tank and had to water change daily until it went down, so just keep up with the testing and be aware that spikes in the quality might happen
Now you might be wondering about this larger tank buisness, so allow me to explain....
Shubunkins are beautiful fish, but they also have the potential to grow up to 30cm in length. What happens when you keep fish in tanks that are too small, is that they become stunted and this can affect their lifespan and health.
The other reasons for large space is for exercise and also to help with the water quality (i.e. too many fish in too small a space, can result in bad water quality for the fish, leading to other issues such as illness, possibly even death).
Like Vic, I've been in a similar position, so we all understand what you're going through
I had two common goldfish in a 21 litre tank, one died within a week because the water quality was so bad and my remaining goldfish Lucky, somehow survived. She now lives by herself (for the time being) in a 90-100 litre/3 foot tank. Because of her time in the 21 litre tank she is badly stunted and so too small to be upgraded to a pond yet.
So for the time being I am keeping her until she is a little bigger, then I will upgrade her to a pond either with me or I will re-home her
For now though, Forrie, just keep up with the water testing and weekly water changes, possibly try doing 50% changes weekly, the more you can change at the moment, the better. I think when Lucky was in her tiny tank I changed around 50-80% each week.
Take your time, to consider what you want to do about the fish
Feel free to ask away with questions too!