Salt will not affect the crayfish but crayfish will attack fish at night time and could be the cause of the damaged tail.
I do not recommend keeping crayfish or big species of shrimp (Macrobrachium species) with fish because the crustaceans are nocturnal and will eat anything they can catch.
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To do a water change just get a basic model gravel cleaner like the one in the following link. Use that to drain some water out and clean the substrate at the same time.
https://www.about-goldfish.com/aquarium-cleaning.html
If you have a huge tank you can make a gravel cleaner from a 1.5 or 2 litre plastic drink bottle and a garden hose or length of clear plastic hose. Cut the bottom off the bottle and remove the lid and throw those bits away. Stick the garden hose in the top of the bottle and run the hose out the door. Use the bottle as the gravel cleaner and drain the tank down. Then refill the tank with dechlorinated water.
As long as you leave enough water in the bottom of the tank for the fish to swim about they will be fine with a 50-75% water change. Just don't chase them around or stress them out and they will usually be fine.
If you make up the new water before draining the tank, then you can fill the tank straight after draining it and the fish won't be in shallow water for as long.[/QUOTE when I do the water change, how do I know If other fish will die like my last water change wear I lost two fish? I’m really trying to be safe here.
