Firstly You should check your water quality, Second is that should be used tap water because tap water should be airatted for 48 hours due to chlorine. You should run a your system for 10 days with cheap fish in it. To get your Bacteria going and You can follow some steps
1. Seperate the larger ones from the smaller ones.
2. Cover the tank with something that will exclude the predator.
3. Hide and wait till the person strikes and catch them in the act.
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