I had about 50 to 60 yellow shrimp in a 5 gallon tank (from an original 18). They were just kind of hobbling along despite the right water conditions until I started adding Kent's iodine-- then they took off! Then I had to go out of town for business on a long trip. My husband didn't know how to care for the tank, so basically left it alone. When I returned a 1/3 of the water was evaporated. I tried just topping off the water with fresh treated water to tie me over until I could get to a serious water change-- then the tank started to crash. The pH plumeted to 6 or below (was registering at the lower limit of the scale). Even the frogbit that completely covered the surface of the water dissolved, as well as my banana plant and half of my java ferns! The only thing that didn't seem to be bothered by this fiasco was my male betta!.... Now a *million* water changes later, the tank has almost stabilized.
I would like to resume having shrimp again, 'cause they're fun and they keep the tank and plants super clean. I am seriously considering a larger tank, though. This size is a bit difficult to manage, temperature-wise.