Xenopus is one of the few frogs where the lateral line system is preserved in the adult stage. This system is found in tadpoles and fish. It allows them to detect food and possible predators in the water. High power filters can cause excessive wave action that can stress out the frogs as well as cause "gas bubble disease", similar to the problem you are experiencing with your frogs. Along with the salt bath, you may want to add an antibacterial such as Maracyn, found in tropical fish stores. Bloating often causes a weakened immune system which makes the frogs susceptible to other infections.