Hello and welcome to the hobby! Your 20 gallon long will work, but keep in mind that with a smaller tank, your options for stocking are limited. You may be able to put in a couple clown fish and maybe 1 or 2 other fish, from what I understand, depending on how well you keep up with water changes. Many fish are not small enough for a 20 gallon. In my opinion, your larger 55 gallon talk would work just fine. Or even the 29 gallon would be okay for a start. Most of the good fish require a larger 55 gallon tank. Keep in mind that the larger the tank, the more costly the maintanence and equipment. But you have a larger window for error and better options for fish. If you've already got 3 tanks there's no need to buy another lol.
Salt water tanks can be expensive with all the equipment. If you go with corals, you will need a good light fixture. You will also need powerheads and a protein skimmer. Live rock, sand and lots of salt. You'll be buying lots of water or investing in a pricey RO/DI filter of your own. How much money are you looking to spend on this tank?
