Although there is a fair chance it might be a tumor, it is not the only possibility. A while back last year i had a goldfish that developed a strange lump like what your goldfish has, and although while at first i thought it was a tumor, the lump later erupted into a more pimple-like growth. After much research it turned out that the goldfish had an uncommon type of internal bacterial infection that can become very infectious as the pimple-like growth later erupts puss which passes on the disease to the other fish in the tank.
Literally days after identifying the disease though another of the goldfish started showing similar symptoms. I treated the tank with "Anti Internal Bacteria" by Interpet and Pimafix, although by the time i started treatment the original infected goldfish later turned out to be too far gone to save, i still managed to treat and save the second goldfish, which made a full recovery
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So just in case your goldfish has a similar condition as to what mine had, i would recommend treating them with "Anti Internal Bacteria" by Interpet and Pimafix just to be on the safe side, doing lots of regular small water changes with dechlorinator/water conditioner and keeping a close eye on their health in general etc. If it is a tumor there's not really anything you can do to cure the fish of it, however it still might live for a long time even with such a condition.
When it comes to fish with tumors, the time to euthanise them is the time when they start to appear obviously negatively impacted by the growth, as sometimes they can live in good health for a while with them. A lot of what depends on how long a tumor takes to kill a goldfish depends on where it is on the body and its proximity to other internal organs etc, and whether it goes cancerous or not. Unfortunately tumors are not that uncommon in goldish, which is largely a result of the intensive breeding/inbreeding many have been subjected to, so you must face this as a possibility.
I have to agree with the others in that in the long run a 30litre tank is too small for goldfish, although i think the main thing right now is to see whether it is possible to treat the goldfish. On the other hand though, even at this point in time if you have other goldfish it would still be beneficial to upgrade the tank to a larger one or just rehome the other healthy goldfish for some smaller growing variety fish etc. Good luck either way though with whatever happens
!