For 2 to 3ft, 1/5 to 1/4 will be good. Though some will use 1/10 for 2ft. Risk is always there for second hand. Check the owner, lol, condition such as the fins, rust, run it to see got noise and leak, etc. Buy from reputable reefers. For such chiller, there is one water inlet and outlet. Place yr pump in the sump(maybe skimmer compartment) and connect the pump to the inlet. Then use a hose to let the water flow out of the outlet to the return compartment of yr sum. For 1/5 to 1/4, pump use can be 2000 l to maybe 3000 but due cater headloss from joint, etc. Hope this help.