I have kept nano tank just for shrimp set up only.
Have to say they were 5 US gals or 20 litres and is the smallest I would go to be entirely honest.
Keeping water parameters in these small tanks can be fairly tricky but the biggest help for these sort of set up is to make the tank densely planted with live plants which helps no end in helping making the shrimps feel more secure and breed more but not so much that it will overwhelm the tank.
7.5 to 10 gals is much more comfortable to do so, obviously the larger and the more established the tank, the more likely that it will be more of a success for shrimp only set ups.
my first nano tank, 10 gals and after a few months of plants growth, had assassin snails and RCS, pretty good little tank.
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Below are 2x 5 gallon / 20 litre tanks, RCS, amanos and ramshorns snails, densely planted and a good little tank set ups that I had running for about 3 years
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