Is it ok to replace all the existing gravel/sand and tank water?
By disposing of all the gravel and putting in all new water you will run the risk of killing off all beneficial bacteria in the tank and making the situation worse, not better. Over-medicating can also have the same effect and kill off all that bacteria too. Instead, you can change the substrate if it’s done VERY carefully and not all the water is removed, I have replaced substrate without too much trouble but have kept half old and half new water and not lost fish as a result. As for all the problems you’ve been having, almost all are related to stress and water quality problems, how often are you doing water changes and did the tank cycle before you added fish? It could also be that you bought diseased fish and brought general problems into the tank from the store. Get your own liquid test kit if you don’t already have one and do regular tests to keep an eye on parameters. The perfect readings are 0 ammonia and nitrite and under 20ppm of nitrates, anything else will be partially resp
