TDS should be about the same in the tank or from the tap unless using r/o or adding buffers etc. It is indirectly a conductivity reading that measures dissolved minerals. the fish will supply some organics that are not really measurable on a TDS meter.
If anything usually TDS would be lower in the tank due to minerals being consumed by the nitrifying bacteria in the filter as they metabolize .
Perhaps you can tell us your GH and your KH to give a more specific idea