I use "Tahitian moon sand" its pure black, rince it real good. What ever sand you use, look for something that doesnt raise the PH of the water or change any water propertys.
Far as i know the Tahitian moon sand is neutral it doesnt do any thing to the water.
Its very easy to clean the sand, every thing stays on teh top and you just siphon it out.
And if you dont change your gravel you pretty much are eventualy going to kill your axolotl. So if you really care for your axolotl change it to sand.
As for filter, depending on the size of your tank, a hang on the back filter or a sponge filter seem like good ideas. Just dont make the same mistake i did, my fitler is too big and makes to big of a current. Live plants also help filter the water too.