Quote Originally Posted by Michael Mutmansky
However, I'm thinking that the calcium oxalate does not have the tendency to form structures large enough to be visibly crystal forms unless there is an extremely large amount of calcium in the wash water, because it appears to precipitate rapidly before it has the chance to collect into larger structures.
Oddly enough, the less calcium there is the larger crystals will tend to form. With lots of calcium it will just go milky with lots of microscopic grains...

"Insoluble" is a relative term. Calcium oxalate is relatively insoluble, but still soluble enough that crystal growth will occur with low calcium concentration. Reducing the oxalate concentration enough to matter is not a viable proposition, you would have to go down to a few ppm to make any difference! It is much easier to eliminate the calcium.