I had a phonme call from my ISP last week about my bandwidth it was over 200GB in 6 days I currently have 2 PCs that were using windows and 2 laptops that are using linux I need to install bandwidth monitors on my Linux laptops but am not sure what to use

It would really be better to do this at the router level. A router flashed with dd-wrt firmware will be able to monitor bandwidth usage. A supported Netgear router with firmware from myopenrouter.com would be my recommendation.

the problem is though that I am stuck using my ISPs router( cisco DPC3825) which I cannot seem to find out where to make it so I can use my asus router which has tomato firmaware.

Can you go from your ISP's router into a subnet on your router and thence to your network?

Assuming you have not down/uploaded traffic in those quantities (that would be several complete multi-year television series), there may be a good chance that one or more of your computers has been compromised or someone is leeching your wireless, if you have wireless, for nefarious ends.

While you work on the monitoring issue, change your wireless passwords and scan all your computers, especially the Windows ones, for viruses, rootkits, and other malware. If you a "guest" wireless network, disable it or put a password on it.