One thing about the rice that can cause problems since it is high in carbohydrates (starches), is that if these carbohydrates can be released into the water column. There have been quite a few scientific studies that have found if the dissolved organic content of tank water is increased, particularly the carbohydrates, glucose, dextrose, galactose, lactose, arabinose, starch & other unisolated carbon sources, that this leads to unbalances in their associated micro-organisms (including bacteria). These unbalances due to increased starch and sugar levels leads to coral tissue necrosis.
This is one study that reflects these facts and also provides additional studies as reference. Please note that everything should be micromolar, NOT millimolar..
There is a nice chart in this article that shows how high the coral mortality is with increasing sugars and starches. If you notice higher levels of starches is more toxic to coral then higher levels of nitrate, ammonia and phosphate.
Role of elevated organic carbon levels and microbial activity in coral mortality
http://phage.sdsu.edu/research/pdf/Kline - DOC and coral death 5-24-06.pdf
From it:
Treatment of corals with phosphorus
or nitrogen sources did not cause significant
mortality in five 30 d experiments
with respect to the seawater controls
(Fig. 1; p > 0.1, Mann-Whitney U-test).
Coral mortality caused by DOC treatments
was on average 5-fold higher
(36.6%), and significantly different
compared to controls (p < 0.001). Individual
treatments that had significantly
greater mortality compared with controls
included 25 mg l–1 lactose (p < 0.001),
25 mg l–1 starch (p < 0.01), 25 mg l–1
galactose (p < 0.05), 12.5 mg l–1 glucose
(p < 0.05) and 25.0 mg l–1 glucose (p <
0.05). Organic carbon treatments
caused pathologies similar to those reported
for band diseases, with a progressive
loss of tissue starting at the colony
margins, as well as rapid sloughing of
coral tissue. Similar mortality patterns
due to DOC loading were also observed
in 4 previous culturing experiments
using a simpler culturing system in
Panama and Puerto Rico (data not
shown). Naturally occurring reef POM
caused significant bleaching of the corals
in both treatments (Fig. 1, p < 0.005).
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This has been my major concern all along and it does appear that two hobbyists are having problems with the starches leaching into the water column. If the release of startches and sugars could be controlled then the rice coudl be a good source for increasing bacterial populations, but if starch & carbohydrates can't be controlled then I believe hobbyists will continue to have problems in with their coral.