The other main reason is that most species have very different care requirements as far as temp and hardness. For instance, neocaridina (cherry, snowball, yellow, blue pearl, etc) like harder water and warmer temps (in the mid 70's and up). Caridina species (like crystal reds, bees, etc) like cooler water and very soft hardness. Amanos are flexible and are excellent algae eaters. Ghost shrimp are different in that they are scavengers so can prey on smaller or juvenile shrimp of other species. Alot of us keep our shrimp in seperate tanks as well as it can be challenging to differentiate between the young and alot of people who keep shrimp, end up selling the eventual offspring ot other hobbyists, lfs, or trading for other stock,e tc.