Another idea--not yet tried--for cleaning these systems. Pour about half a
cup of bleach into maybe half a gallon of water left in the tamk. Put a tube
on the faucet that recirculates into the holding tank. Turn the faucet on
and leave it to recirculate for maybe 20 minutes. Flush it all out.

You always wonder what kind of cr*p the PO left in there for you, at least
we do...and 23 years of it...