we have little sliders come in every spring but, usually go back to the creek come fall..I've always been told they carry parasites and I'm sure they may...but, we've never had any problems with them..I did have one that stayed around for a few years...I named him and would take him into the greenhouse pond for the winter and somehow he always found his way back to the outside ponds during the spring...he would go from pond to pond--I'd never know where I was going to find him..he ate koi food right there beside the koi....but, the last year he got to where he was tormenting my big asagi female by nipping at her fins and I was afraid he would do damage to them---only her and no one else--so i did have to relocate him to a local pond..Just watch him and see how he is acting with the fish...
Lawanna