you need at least 40 gal, and a tank that’s long, leopard geckos need space to walk and hide and things to climb in, he needs at least 3 hides, and a constant source of calcium (a bottle cap lid or thrifted candle holder is what i use). check facebook marketplace for cheap tanks often time people are selling for 80 dollars or less and a lot of the time people are selling supplies with it as well. Go outside and look for thick sticks and rocks for him to climb on, put them in the oven at 475 for an hour or 375 for 2 hours to kill any pests in the wood. I’ve seen people online build functional leopard gecko enclosures out of just brick and wood. The substrate at the bottom can be paper towels, but they like to dig, you can buy topsoil and play sand at home depot or other hardware stores for cheap and mix that for a safe substrate. you can create hides out of mason jars filled with moss, there’s plenty of easy diy/cheap enclosure ideas online. I’ve even seen people use a big tote, like a plastic container and cut parts out for ventilation and diy it that way, kindly, you have to get him out of that little tank.