Jen wrote:

So what's the general consensus on using undergravel filters? Are they
good or bad?

They work and are still a reliable filtration method, but somewhat higher
maintenance. I used to use them for many years. I don't think I'd use them
with goldfish. Just too much waste. With an external filter, like a
canister (or HOB or wet/dry) you get the wastes out of the tank into a
place you can easily clean up, and with goldfish you need to do that a bit.

With other fish it's not so bad, but the same applies. If you are willing to
do very regular gravel vacuuming and maybe break down the tank every couple
of years, then UGF is fine. It's a good biofilter, but you can replace it
with enough media in some other filtration system.

It's a personal choice.