Looking for Downloaders on network.

Hello community.

I buy Mikrotik GW specially for manage our small network. We have like 20 peoples on LAN and some of them use too much of traffic. We have for internet limit on traffic per month = 600GB
Last 2 months users spend all traffic in first 15 days and than all of them complain “internet is not working” and so on… So that’s why my hunt need to start.

Can someone support me with working solution / script how to monitor traffic per LAN IP with possibility to reset counters every month?
Script for traffic monitor + maybe email notification will be great.

I bet someone already needed and used something like this.
Thanks very very much.