A fluid treatment tank for an aeration filter has an air blocker separating a chamber in fluid communication with a control valve of the filter from a pocket of air above the liquid in the tank and the control valve. The air blocker contains a volume of liquid that acts as a trap that prevents or at least inhibits air in the pocket from backing up into the chamber. Contact between the control valve and air is dramatically reduced. The air blocker may comprise a baffle that directs any air flowing through the air blocker downwardly and then back upwardly before the air can escape the air blocker.