For a 20 gallon tank you can find a nicely built cabinet or small end table to blend with your home decor. Not a big fan either of most manufactured cabinets as they are built for function and that generic look is designed to be neutral to fit in the most consumer homes.
I have a 55 gallon freshwater tank on an arts and crafts sofa table I picked up in Michigan from an a mid-range furniture store 25 years ago. It's solidly built even though it's manufactured wood but it has been holding up that tank for 4 years now so stress test complete. I've even moved it on sliders a few times. Here's an image similar to mine
Couple of caveats here of course. This is your risk of course. All cabinets are really, even the aquarium manufacturer's models. 20 gallons is only about 160-180lbs all together so anything that feels built solid is likely fine. Manufactured wood (MDF) is fine as long as it's finished well and 1/2" to 3/4" inch thick. Stability is a key if it wobbles at all - don't use it. I could dance on that sofa table and it's not moving, I haven't but I could. Manufactured wood has a bad reputation but for yours (and my) uses it's fine and dandy if chosen carefully.
I've gotten the top of mine wet - just wiped it off with no issues other than a little staining which just adds character, right?. Make sure whatever you get doesn't have joints that water could seep in easily with minor maintenance spills and have a rag handy.
I'd stay away from anything less that 15" wide - which lets most bookcases out. 160 lbs isn't all that much until it's on top of something thin, then it's going to be top heavy and tippy. But if that bookcase as solid footing wider than the top, your back in business.
Good luck, update us on what you picked.