Are you mixing the water up at least a day before using it. If not, that will make a difference. Also check your tank water pH and alkalinity. Is your water tap water or is it RO water? If your water got worse following the water change it could be your water source, or if the salts are not totally dissolved or your pH is off you could have particulates or even precipitates in your water that would clear with simple changes in methods. Replacement water should be mixed up at least a day a head and an air line or circulation pump should be in the water until it is added to the tank, it should also be very close to the same parameters as the tank water as far as salinity, temperature and pH. Hopefully your not using tap water. If you are then you might have to buy water or get a filter for your supply water. Regardless of your water source you can probably get your local fish store to check your supply water for total dissolved solids, pH, hardness and if your lucky phosphates. Or if your using tap water check with the company who supplies the water as they have to make public their water test data, that should give the above data plus tell you if there are any heavy metals present.