What your doing is exactly what you want to do. Water changes are ESSENTIAL to the health of your fish. Eheim does make a gravel vac like you are looking for, but it's never going to remove the nitrates that build up in your tank or filter out all the fine particals. If your concearned about removing too much water then just do one half of your tank this time and the other half the next. But if you can do the entire tank with only removing 25% of the water then that is perfect.
What your doing is exactly what you want to do. Water changes are ESSENTIAL to the health of your fish. Eheim does make a gravel vac like you are looking for, but it's never going to remove the nitrates that build up in your tank or filter out all the fine particals. If your concearned about removing too much water then just do one half of your tank this time and the other half the next. But if you can do the entire tank with only removing 25% of the water then that is perfect.
how big is the tank and how much water are you changing at a time?