Stiring the gravel up and releasing a lot of dirt into the water can make fish sick but doesn't normally kill them unless the water smells of rotten eggs after stirring the gravel.
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You should only do a 75% water change once a day, if you are doing daily water changes. There is no need to do it 2 times in a row on the same day.
If the fish are dying after you add new water, there is something in the water that is affecting them.
You should check the tap water for ammonia, nitrite, nitrate, pH and GH.
If the well is on your property, make sure nothing has fallen in and contaminated it.
You can get water tested by an independent lab to see if there is any harmful chemicals that might have leached into the ground water.
Until that gets done, I would recommend putting the new well water into a large bucket and aerating it for 24 hours before adding it to the tanks. This will help get the dissolved gasses in the water back to normal levels.
If the fish still die after using aerated well water, you should add a carbon filter to the container of well water and leave the carbon filter running in the water for at least 24 hours. This will help to remove any chemicals or heavy metals that might have gotten into the well water.
If the fish still die after filtering the well water through carbon, and aerating it for 24 hours, you need to get the well water tested to make sure it is safe for you and the other animals and fish you keep.
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If you don't have a gravel cleaner because yours broke, don't gravel clean or stir up the substrate until you get a new gravel cleaner. I recommend doing a 75% water change and gravel cleaning the tank once a week to help keep the tank clean. However, if the gravel cleaner breaks or you can't do a water change that week, it is normally fine to miss out on the water change or gravel clean for that week.
Doing the water change and gravel clean once a week helps keep the tank in great condition so you can miss a water change once in a while if you have to. Don't make a habit of missing water changes but missing one every now and then is not a big problem if you normally do them every week.
You can make a gravel cleaner out of a 1, 1.5 or 2 litre plastic drink bottle.
1) Cut the bottom off the bottle and throw the bottom bit away.
2) Remove the lid and plastic ring on the top of the bottle and throw those 2 bits away.
3) Put a garden hose in the top of the bottle and run the hose out the door onto the lawn, and you have a gravel cleaner. If you don't want to use a garden hose, take the bottle to a hardware store and buy a couple of meters of clear plastic hose to go on it.