I've been in freshwater for about 10 years, and would like to experiment how it feels to have a salt tank aswell. I'm going to use my old 20 gallon tank, but I have a few questions: Can I use my HOB filter that I already have or do i need to but a specific filter, would a sponge filter still work in saltwater, good fish to stock my tank with, any extra equipment I have to buy to start off, and if sticking with gravel okay or do I have to switch to sand? I've heard you need to move sand every so often for some reason, is it necessary? I'm sorry for my newbie questions, just don't want to waste money on anything I don't need :/