The concentration of my constituent in the pipe that is directly downstream of my tank, which is my concentration source for the model, shows the concentration increasing and decreasing in a spikey manner at the stop node, but the concentration should be the same as the starting concentration at the tank. Why is this happening?
This issue might occur if the concentration source type of tank is set to "Concentration" . Yes, this source type fixes the concentration of any external inflow entering the network, but it only does so for a flow from a reservoir or from a negative demand placed at a junction. For a tank you will want to use the constituent source type of "setpoint booster", "flow paced booster", or "mass booster". You can read more about the differences between these source types in the wiki article under the "See Also" section below.
What is the difference between the different constituent source types used for a constituent analysis?