It's not normal for adult axolotls to swim a lot. If they are swimming all the time, it's stress. The tank really is too small for 2 adults. I wouldn't put 2 adults in anything less than a 20gallon long (76x30x30cm).
If you don't think they're eating gravel, put them in a bowl for a couple of days, and see how much gravel they poo out. They're eating it, whether you think they are or not. Try doing a forum search for other people who have done this. Some axolotls have retained gravel for 2 years.