I'm saying that in a closed system mass should stay constant. The gain is during the filtration process: more water comes out than in. I guess it makes a little sense that some food may be converted to water but I'd assume food stored for space travel would be dried to reduce volume. I found it to rapidly create water in a way that seems impossible, especially considering solid waste contains water too. At any rate I noticed the error when creating a greenhouse mod to work with TAC specifically, and balancing water / oxy /food intakes for Kerbals. The problem arose when the water filters stopped due to full water tanks and failed to produce enough waste for conversion to biomass. While it makes sense that solid / liquid gas waste gets mixed up in bodies, for simplification purposes I like to keep conservation of mass between the three. Basically i'm saying a filter should, logically' produce the exact same amount a of water a kerbal 'peeing' into it consumes, at least not more.