The tank is my wife's hobby, I just "get" to help clean it.
I suggested to her we just mark the level, remove a gallon carefully and measure the difference. She wanted a math solution and I knew where to ask the question.
We also have a koi pond in the back yard. She "lets" me help clean the filters on that as well. The Koi pond has a direct fill from the water well so it is a super simple thing to add water.
Thanks all.