Wilder Im going to put them in there for a while so then I can get a bigger tank. Would 90L be ok with a 60g external to go with it + the undergracel already in there?
No, fraid not. If you keep the in a tank that size, their growth will be severely stunted and they will die a young age either due to some organ failure, or whatever pathogens happen to be in the tank when their immune systems reach an all time low.
Goldfish get pretty big, I don't know if you have ever seen big ones, but fancy goldfish like yours could easily get to be 7" ball shaped fish.
When you keep them in small volumes of water, metabolites build up fast, stunting the fish. Also combined with other things building up in the water, the fish's liver has to work very hard and if things get very bad it can fail.
This post explains it.
A bigger filter will not solve this (but would still be needed to deal with the huge amount of ammonia that goldfish produce). Extra water changes would, but you would end up having to do big daily water changes - and you would have two huge fish with no room to move.
So unfortunately if you care about the fishes welfare, you are either going to have to get about at the
very least and 120 litre tank with an external filter rated for a bigger tank, or rehome the fish
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