Kelsey, does your other tank have an established cycle? And are the fish in there healthy?
If so, and maybe you already know this, but I *think* you can drop the new filter in with the goldfish (not removing their established filter, just adding the new one) and it will build the good bacteria much faster.
Like I said, not 100% sure, but I thought I've read that of you want to quickly chucky a new hang-on-back filter you can add it on an established tank to speed the process.
I dunno, just something to think about &/or research.