"David Young" wrote in message
news:zPUhd.346600$MQ5.261894@attbi_s52...
Marc,
Nope, no additives. Auto water top off system of RO-DI. Salt levels
adjusted
as necessary.

Try checking your RO/DI water. DI resins are called "mixed" resins. One
resin replaces all cations such as Ca2+ and Na+ with acidic H+. The other
replaces anions such as Cl- with alkaline OH-. In a balanced resin the H+
and OH- neutralise each other instantly to form water. However resins can
become unbalanced for various reasons. The efficiency goes down and the PH
swings one way or the other. The PH from your DI resin water could be high
and has basically destroyed your tanks buffering capacity over time.

The way we can tell when a resin has gone bad in an industrial setting is
because the water smells fishy. Of course that doesn't work with an
aquarium :-)

Robert