You can get rubber mats in a variety of sizes for snow blowers, laundry machines, heck even cardio mats for home gyms. BUT I wouldn't consider one for under a tank because no matter how big, it will have an edge. If water wicks under that edge then the result will be 100X worse then if it just was on regular hardwood floors and left to dry on it's own. Water under a waterproof mat will stay wet MUCH longer. As someone who has 2 tanks in a room with hardwood floors, my advice is to just have a bunch of towels you can use whenever you need to. If that's your regular bath towels or beach towels then fine. But make sure you have a bunch. Then use them liberally after/during water change or any time there is water getting onto the floor.