I would not topoff too much, leave a little space. I have noticed that my sours change their mass/viscosity a little, in the spring the liquid comes close to the bung and in the end off fall and winter the fill line is lower closer to the bottom of where the neck starts. I have no idea what causes this and have wondered for years, since the first one blewoff a little through the airlock. That was after topping off 9mos after the bug pitch. Now i top all my sours right to the bottom of where the neck starts. i would not topoff with water however, because a watery mouthfeel is allready an issue with the brett and lacto eating all the dextrins. Might be ok in a lambic because the pedio makes a fuller mouthfeel but i would avoid it. I topoff with extra starter wort that i save from last runnings. I actually have found that the brett will make more funk if you feed it dextrinous wort a year after pitching. (See SOLERA). If its funky, i would avoid topping off. Blending is allways an option, i blend most of mine.