Shrimp

Freshwater shrimp. In 1983 Takashi Amano first introduced into his natural planted aquaria a species of Japanese freshwater shrimp, known then as Caridina japonica but since 2006 identified as Caridina multidentata. Hip American aquarists like George Booth began to be interested in freshwater shrimp around 1994, first as detritus scavengers and algae-pickers, but nowadays, more and more for their own sakes.
 
Finding a genuinely freshwater species had been impossible for me in New York until 2002. We were still mostly being offered local temperate-zone brackish water American species of "glass" or "ghost" shrimp; they don't survive in completely fresh water for very long. The previous year I had five thriving in a tiny unheated (dipping below 50^oF) lightly brackish windowsill aquarium with crushed coral substrate and algae and Java Moss, to see how they'd survive. They lasted six months. Not my usual ecotope...
 
Amano's Caridina multidentata, a native of Japan, tolerates pH down to 6.0, but is not tropical, so the water temperature has to stay below 30^oC. It also returns to brackish water to reproduce, so doesn't establish natural populatioons in our aquaria. Six other species from the families Atyidæ and Palæmonidæ have been tried out in aquaria. Some of them have been passed off as "Amano's shrimp." Some are deeply unhappy outside their native brackish water, others like Macrobium spp, are neither algae-eaters nor especially helpful, but interesting maybe in their own right.
 
Atyopsis moluccensis. One of the Atyidae is the "Singapore shrimp" or "Wood Shrimp," Atyopsis moluccensis ("from the Molucca Islands"). Once these big filter-feeders are at home, feeling mildly territorial, clinging to a large leaf or a piece of wood where the current is strongest, they'll color to a bright strong rusty wood color that even approximates flaky bark and waterlogged wood grain. A white streak runs down their back.
 
The "Cherry Shrimp" is a carefully selected aquarium form of Neocaridina heteropoda from Taiwan and neighboring South China, ordinarily inconspicuously brownish and transparent; it has been selected for a vibrant color that would eliminate it quickly in a natural ecotope. A yellow variant has also been bred, and other colors are coming, Robert Paul Hudson blogs at AquaBotanic, 2 March 2011.  Its eggs are tinted to match, visible within the bigger and briohter red female;  they hatch before being ejected as miniature versions of their adult selves: there is no larval stage for these shrimp. You can expect it to reach about 1.5 inches.
 
Neocaridina is a sociable algae-eater that doesn't scrap with conspecifics, and may even be considered gregarious. It browses on the biofilm and sifts through detritus. I'm afraid that it would eat fish eggs.  I wouldn't trust a lively barb or rasbora with them, either; mine are safe in a densely-planted Corydoras panda species tank that doubles as a plant nursery.
 
The species Neocaridina heteropoda, which had been considered simply a Chinese population of Neocaridina denticulata (N. denticulata sinensis), was separated out as its own species in 2002.
 
Freshwater shrimp vary in the intensity of their color, depending on the light they are receiving, on their sense of security, and on feeding and water conditions. Most shrimp photos that turn up at websites and in the hobby press show these animals at their most splendidly intense. All crustaceans molt. When a shrimp is ready to molt it will become inactive and withdrawn for a couple of days. You may not even be able to find it. I've never caught mine in the act of splitting its carapace crossways across the back behind the headshield. The first time you see the cast shell you'll think your shrimp has died.
 
Twice I have lost founder populations of Cherry Shrimp in a planted 10-gallon tank that seemed to be ideal.  After a couple of days they would lose coordination, swimming erratically in loops and agonizing in spastic contractions before they died among the plants.  I had forgotten a long-ago medication for Ich in that aquarium, using a commercial preparation that included copper sulfate. At my pH levels below 7.0, the copper was still coming out of its precipitation on gravel after all that time, and killing the shrimp.  The tank is permanently off-limits to shrimp now, but it took two mass mortalities to teach me. PolyFilter or its equivalent in the filter is a good protection from low copper levels that may also be in your water supply, as it is in mine.
 
Links. More detailed information on these species, brought together by Joe Anderson, is now archived at The Krib. The reproduction of Caridina and Neocaridina is explained, with useful close-up microphotos, at PlanetInverts. You can find more shrimp info at Robyn Rhudy's ever expanding pages, which look into all the life in her ponds. Frans Goddijns' diary notes have some details about the shrimp in his planted tanks, mostly Caridina.  There is also a hobby-oriented book on this subject: Uwe Werner, Shrimps, Crayfishes and Crabs in the Freshwater Aquarium.
 
"Cherry Shrimp" at Wikipedia. Neocaridina heteropoda at AquaticCommunity.