First off you need to calculate 2 things.
1st, How many gallons per hour you need to reach the tank
2nd, How many gallons per hour your chiller will need to run.
Once you have those 2 number then you nee dto match a pump that can deliver more than that becaus eyou will lose a lot in head pressure going up to the tank.
For example Cor 20 is 2000gph at 0 ft. If your tank is about 5ft where the return is you are only getting 1400-1500 gph depending on how may 90 degree elbows and how its routed. in perfect straight pvc pipe with correct diameter it will likley deliver 1400-1500 gph.
If you split that to a chiller that also needs say 700gph, your pump is already starting to look small for the job.
in this example if chiller is taking 700 gph your return might be 600-700 gph at best.