Atya gabonensis, also known as Cameroon Fan Shrimp or Viper Shrimp, has the potential to reach 6". I have only seen and kept 3-4" specimens. They are completely harmless to other aquarium animals and plants. They are filter feeders and feed on algae and microorganisms suspended in the water.
They can be somewhat of a challenge to keep, the first 3 died on me, as long as you meet their requirement they are fairly easy to keep. A well-established aquarium is a must. they also love good cover so I recommend a densely planted aquarium with several hiding places. It's also important to have decent current in the tank. I fed them sinking pellets, liquid fry food, and other fine foods. I squirted liquid fry food using a 10mL syringe into the water near them.
Make sure you don't keep this guys with aggressive tankmates, they are very passive. Also remember to be careful with medicating your tank. Medications containing copper are deadly to shrimp and other invertebrates.
I agree with Payara. I don't suggest these animals put into new tanks. A well established and clean aquarium raises your success to provide these cool animals a safe home away from their home. :) If you see your Atya starting to scavenge from the bottom it is a good indicator that it is not feeding well. All the food kinds that Payara has mentioned work very well for this species of shrimp.
i agree with the two of you. this is not recommended for begginers. this needs a densly planted tank. i place 5 of them in a tank with full of water sprite and java moss. a foam mat filter was placed for the less current filter