I've got an old undergravel filter system that came with a used tank I just bought. I'm not really sure how to properly install it or if it's even worth using compared to hang-on filters. Should I bother setting it up or just get something newer?
Undergravel filters can still work fine for a 20-gallon tank if you set them up right, though they're definitely old school compared to hang-on filters. The main advantage is that they use your entire substrate as the filter media, which gives you decent biological filtration. The downside is they're a pain to clean without disturbing everything, and you can't easily adjust flow rate or add media like you can with modern filters.
If you want to use it, here's the basic setup. You'll need to remove everything from the tank first. Lay the undergravel filter plate flat on the bare bottom - it should cover as much of the tank floor as possible. Attach the air pump or powerhead to the uplift tube (the vertical pipe). If it came with an air pump, you'll run an airline from the pump to the tube. If it has a powerhead, that's an electric motor that sucks water up instead. Then spread your gravel or substrate over the whole plate - you need at least 2-3 inches to get proper water flow through it. The filtered water comes back down through the substrate and returns to the tank at the top of that uplift tube.
Honestly though, for a 20-gallon tank today, a simple hang-on filter or small canister filter is going to be less of a hassle. You get better water flow control, easier maintenance, and you don't have to tear apart your whole setup when you want to do a gravel cleaning. If the undergravel system came free with the tank, it's worth trying if you don't mind the extra work, but don't feel obligated to use it. A basic hang-on filter does the job just fine and lets you actually enjoy the tank without constant fiddling.
If you're going to use it, the biggest thing folks mess up is not rinsing the gravel thoroughly before putting it in - you'll get a cloud of sediment that takes forever to clear and can mess with your cycle. Are you planning to keep the same gravel from the used tank, or starting fresh? And what kind of fish are you thinking about, since that affects whether an undergravel filter has enough flow for their needs.
They're worth setting up if you've already got one lying around - no reason to trash a perfectly functional piece of equipment. The key thing nobody mentions enough is that you need an airstone or powerhead on top to actually move water through the gravel bed, otherwise the whole system just sits there doing nothing. Once you get that running, rinse your substrate like crazy beforehand so you don't spend a week staring at murky water, then layer it evenly over the filter plates and you're good. It won't be as convenient as a hang-on filter, but for a 20-gallon it works fine if you're willing to put in the minimal effort.
Make sure you've got an air pump hooked up to it - that's what actually moves water through the gravel bed, and tons of people forget this part and wonder why their filter isn't doing anything 😅 Rinse your gravel like crazy before adding it, or you'll have a milky tank for days. The thing nobody warns about is that undergravel filters can develop dead spots if your substrate gets compacted, so you'll need to gently disturb the gravel every so often during maintenance to keep water flowing evenly. For a 20-gallon it'll work fine, just maybe pair it with a small hang-on filter later if you want extra flow and mechanical filtration.
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