Mature tanks... how mature?

I'm on the 60 side of the spectrum when I have the chance but I will and have put shrimp in a tank if I use parts and pieces from another tank to seed it after 2 weeks. I do not recommend it though....

I have a 55 going on 4+ months with nothing but snails so far...

On the cherry barbs they can and will take down adult shrimp piece by piece unless you have a lot and I mean a lot of hiding spaces and even then they will hunt them down. I can speak on this as I put shrimp in my barb/dwarf cichlid tank for live food once in awhile.....
 
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