holding tank vents
 

Just an observation to support a point that Peggie made. I just removed
my holding tank and both vents were plugged, they are at the top of the
tank, port and starboard sides.I have to assume that when the tank has
been pumped out at various marinas that the joker valve allowed air in
to the tank to stop the tank collapsing.
Any regulations relating to the use of ABS below the waterline?