Got a little more time.
(One hint: If you can do it without an elbow, you probably should. They kill flow, and fairly often there is another way...two 45º's or even a "long radius" 90º...etc.)
Let's assume:
- a total of 3' of horizontal @ 1"
- services two 1" outlets via non-reducing (full flow) Tee fittings
- 600 GPH of actual flow required at outlet one
(is the chiller currently chilling the sump or the tank?)
(any idea what the head loss is for the chiller?)
- 200 GPH of actual for outlet two
- two 1" ball valves
- services one 3/4" outlet to the display tank
- 1' of vertical @ 1" to connect to the display
- One 1" 90º elbow
- One 1" to 3/4" reducer
- Another 2' of vertical @ 3/4" to reach the top of the tank
- services two 3/4" outlets via...
- one non-reducing (full flow) Tee fitting
- the 3/4" loc lines are full flow tips/outlets
- loc line counts as 1' of horizontal
(if you're using fans or any other style of outlet, you'll have to recompute. 
I'm pretty sure no calculator is going to account for the multiple outlets at multiple heights via multiple pipe diameters. We'll still make the best estimate we can.
Data from freecalc.com pasted in at bottom. I added the upper and lower section head loss together and rounded up to 6' and used the requirement of 600 + 200 + 500 = 1300 GPH (4.9 L/h) of actual, total flow.
It looks like your pump is somewhat comparable to a QuietOne 4000.
I think a QuietOne 5000 or 6000, would make an adequate upgrade. Both are around $100 at marine depot. Plus they are so quiet you'll love them.
Really, really quiet. I currently run a 4000 return...which replaced a 2200 return.(..which actually alternated with a Mag 7.)

My 4000 is barely audible even with all the other equipment off during maintenance and with my head in the sump.
On that note, I think the Mag 18 and 24 would give similar flow rates and are probably stronger motors (higher wattages), but aren't quite as silent and come in at $150+ and $250+. Worth it, depending on your priorities.
Both of these pump lines have a long track record. Definitely recommended for the return flow application.
If you really wanted to stay with the line of pumps you have, the 10000ll appears to fit the bill, but has cost similar to the Mag 24. I also think it's possible you might want to run the 12000 so you wouldn't have to run it full speed, but that depends on what you want and the mfgr recommends.
Unless your tank runs hot due to lighting, I personally wouldn't worry about power use by your return pump....at least not much.
My 2¢!
Liquid Friction Pressure Loss
Pressure Loss (psi): 1.13
Head Loss (ft): 2.6
Line Number: 700 GPH to the lower-tank outlets
Date: 7/23/2016
Nominal Pipe Size: 1
Pipe Schedule: SCH 40
Flow Rate (gpm): 14
Viscosity (cP): 1
Specific Gravity (water=1): 1
Temperature (F): 85
Pipe Roughness (ft): 0.000016
Actual Pipe ID (in.): 1.049
Fluid Velocity (ft/sec): 5.2
Reynolds Number: 42208
Flow Region: Turbulent
Friction Factor: 0.022
Overall K: 6.23
Piping Length (ft): 3
Standard 90 degreeThreaded Elbows: 1
Tee Flow Through: 2
Ball : 2
Pipe Entrance: 1
Pipe Exit : 3
No. of Reducers: 1
Reducer Outlet Size (in) : 0.75
Liquid Friction Pressure Loss
Pressure Loss (psi): 1.16
Head Loss (ft): 2.7
Line Number: 500 GPH to the dispay
Date: 7/23/2016
Nominal Pipe Size: 0.75
Pipe Schedule: SCH 40
Flow Rate (gpm): 10
Viscosity (cP): 1
Specific Gravity (water=1): 1
Temperature (F): 85
Pipe Roughness (ft): 0.000016
Actual Pipe ID (in.): 0.824
Fluid Velocity (ft/sec): 6.02
Reynolds Number: 38381
Flow Region: Turbulent
Friction Factor: 0.023
Overall K: 4.75
Piping Length (ft): 3
Standard 90 degreeThreaded Elbows: 1
Tee Flow Through: 1
Pipe Entrance: 1
Pipe Exit : 2