Saltwater is inherently a little more expensive simply because the water is more. A fish only (FO) isn't much more. Fish only with live rock (FOWLR) moves up a step because of the rock. Reef is the most expensive because of lighting, extra filtration, and the inhabitants. A twenty gallon isn't too pricey to set up in any of these groups, though it can be, depending on what you want to keep. I would scan the getting started threads and see what others have done and gone through, but also read the articles here on aa. There are some great ones. One thing to keep in mind is that marine animals evolved in a very stable environment and thus are not very tolerant of water parameter changes. So, a smaller tank requires more diligence. There is contention on whether a skimmer is required for a small tank, but if you wish to keep corals or other sensitive invertebrates, I would recommend it. Whichever route you go, welcome! And good on you for wanting to research before buying anything!
