I just answered this for a customer recently -- according to our shrimp aquaculturist, for a twenty gallon tank with good filtration and good water changes, one should be able to keep 60 to 100 one inch shrimp without much issue. Any more could be problematic. Basically, five shrimp per gallon should be a normal stocking density providing there is good filtration and water changes are made regularly.
If you 're speaking of fiddlers, which are readily available from the Aquaculture Store, you don't keep them in water. They will live for several weeks in water, but actually they live in the sand right along the shore, so keeping them where they have both a dry area and water is what they need. I've tried a large flat bowl (turtle bowl?) with some rock islands. You can keep 12 comfortably in that bowl. You can also take a large, flat container and fill it with several inches of sand, then sink a little plastic container as the "pool". Keep the sand damp with 1/2 salt water, 1/2 RO/DI water. The fiddlers will bury in the sand.
In both cases we're talking about keeping them in a separate area from your tank.
If you have ocean crabs, such as hermit crabs, you can put quite a few of the small ones into a tank your size.
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