In another forum, I've read posts by an experienced shrimp keeper that has a tank just like what you described. The tank bottom is covered with brown oak and maple leaves that have been soaked to leech out all of the tannis and harmful substances. He has not done any feeding or water changes for some time (months) and the shrimp are reproducing. He states that the shrimp population is not growing as quickly as the tanks that he feeds and regularly changes the water, but they are still healthy.
FYI, the shrimp don't actually eat the leaves. They eat the microbes that eat the leaves.
