Gars!!!!!

I have pumped out my stock tank three times and each time I remove some gars. The last time I pumped it dry was just over a year ago and it remained dry for over three months. I thought everything would be good. I put some tilapia and Golden Shiners in it this past spring. Thinking I could pull some out I have pumped it down and pulled out over 18 gars and nothing else besides a couple of copperheads. Where did they come from? How do you git rid of them? The only water that might run into is from a well or rain run off. No creeks or streams.

The biggest one weighed over 4.5 lbs. That seems pretty large for less than one year growing time.