There's a lot of important bacteria that is essential to the bio filter living in that sponge - don't throw it away! If you replace it, you'll be pretty much starting over cycling your tank. The snails would probably end up taking over the new sponge, too. I don't have snails, so I don't know a good way to get rid of them. Maybe someone else here could give you advice on that.
With the water change, you can remove enough water to leave the fish covered and then refill it with fresh. Vacuum the gravel real good while you're draining it. So much food and waste gets trapped in gravel and it's a breeding ground for unhealthy bacteria. Test your water. If ammonia and nitrite are staying at zero and the nitrates don't exceed 40 ppm, you could probably change it once a week. If you ever see ammonia or nitrite, do a big water change since these are very toxic. If you find that nitrates are rising a lot - over 40 ppm, you should change the water more than once a week. The only way to know for sure is to test the water.
