It sounds like you may have a mineral imbalance lowering the specific gravity of the water below what the freshwater scuds are accustomed to in a balanced natural spring water profile, possibly causing stress or causing ruptures to the air bladder. I am not a fan of reconstituted osmotic water for freshwater environments as is the standard practice in marine aquaria. Incidentally, these are actually Hyallela azteca not gammarus the name commonly used for many amphipods. I recommend using spring water containing a natural balanced mineral profile which will be in a better balanced asimilable form for the scuds. For the setup I recommend using crushed coral substrate and an overgrown planted aquarium with either duckweed or Asian water meal on the surface and an overgrown mass of Naja grass surface to substrate in the entire aquarium as this is a great nitrate buffer in a low oxygen environment. Filtration is optional and minimal aeration is best, no current. Bubbles breaking the surface every 1 or 2 seconds apart is all that is needed.