I have a soft spot for the Rena XP filters. Buying new is fine, I have been able to find some of these used and part them together to make a working filter for less than half of a new one. I replaced the hoses with new line from Lowes too, so I'm not stuck with old, dirty hoses either. Parts are generally available too if needed. If you buy used make sure to check the impeller, it should not look worn or uneven - they will run like that for awhile too but eventually will need replacing. Don't undersize the Rena either, it's a good filter but if you're going to buy one, new or used, go for the biggest you can. I have run a 10 gallon on an XP1 before and defused the outflow with a sponge. I do tend to agree that your goldfish would probably enjoy a bigger tank though, although you didn't mention its size either. Since they do tend to be messier fish, and especially if you think you might upgrade, I'd start looking at a XP4.