Personally, I wouldn't keep a bamboo shrimp in anything less than a 29g. I have heard of some success in smaller, tanks, but its easier to keep them happy in larger tanks because of the current as was previously stated. Bamboo shrimp are obligate filter feeders, they are also very sensitive to water quality issues.
Do you have a test kit to check your water parameters? I suggest doing a pwc. Are you using any sort of supplemental invert food? To live in a tank that small, they will need supplemental feeding in addition to the spare bits they naturally catch in the current.
Some bamboo shrimp are suicidal escape artists too. So, some will try and climb out of the tank even in good conditions. I had one who always tried to climb out of my tank and would just sit above the water line sometimes even though my params were great and he was getting fed. He still lived a nice, long shrimp life despite this habit (my tank was well-covered).
Having enough hiding places can curb this behavior in most individuals, but some will always try to climb IME.
Another thing to note, although you may well already know of this, is that bamboo shrimp are wild-caught critters and a lot of them are often rather unhealthy from improper keeping before they make it to your tank. Even if all your conditions are perfect, they can be hard to keep alive. Also, there is no way of knowing how old they are at the time of purchase, so they may already be very old.
Good luck with your little guys. Let us know about the water parameters and if anything changes.
