Which would be equivalent to what size mate? Ive based the size requirements on the size of the tank they have been in for the last 3 years. over a year of that aswel in the hands of somebody who didnt look after them at all and....youll love this bit....had 2 common pleco in there aswel!! I have since stolen my tank back, re-homed the pleco (who were already the full length of the tank!) But Gerard and Owen seem quite happy in their tank on their own lol, not trying to justify it just letting you know the scenario. If you say its not suitable then ill listen.
A 20gal is usually between 2.5-3 feet long, unless you find one that's taller. Here in the coldwater fish section it's recommended that they have 20gallons per fish and another 10gal for every other fish.
Thing is with goldfish is that that are basically carp - they can get very big, 8-10" easily and up to about a foot.
They live a long time (up to around 40 years, expect a minimum of 20 years if you look after them correctly and nothing goes wrong) and grow quite fast, though it does appear that (unlike a lot of other species) they can be stunted significantly in small tanks and this eventually kills them, if I remember correctly I think it has something to do with the build up of metabolites and other waste products in the water that damages their liver.
Being big messy carp, they pretty much demand an external filter too, although if you wanted to use several internal filters it would probably do, you would need to clean them out a lot more.
It's a shame about goldfish, they are a very abused species. Even the people who actually want yo look after them properly often don't have the space (I had to rehome my goldfish years ago).
What I would recommend doing is either getting a 20+ gallon tank and an external filter (picking these up second hand should cost too much), or finding someone with a decent sized deep enough pond that will take them. Obviously, releasing them into the wild is not an option (just had to mention that as I've seen a few that have been, they can damage ecosystems over the short term, and are usually eaten by other fish or herons due to their bright colours anyway)
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