That's a huge pond and I'm sure they would be happy there since it's established and all.
But if you do decide to upgrade I'd definitely go 75g+. 75 g is the same length as a 55g (as is a 90g) just deeper front to back. I've bought LOTS of fish tanks you just need to search around for deals on used tanks. You wouldn't believe how cheap people will sell them when they just want them out of their house. Heck, I've sold tanks myself dirt cheap before just because I had no use for them anymore.