The pumps I prefer are the Digital Brushless units. In the brushless line the most commonly available units are model numbers 7523-60, 7550-30, 77301-20, and 77301-30. The 77301-30 controller is in a waterproof case. The rest of them will need to be kept free of moisture and salt creep as they are either in a modular (motor and controller separate) or a console (motor and controller in the same case).
If it is a modular system you can easily tell if it is brushless or not. The black ports circled in red are for the brushes. Brushless units will not have this.
They have a couple other variations that change things such as cord length, operating voltage, color, and rpm range so don't let those model numbers necessarily be your only search terms. Often they are labeled as a Barnant if custom ordered. A simple phone call to Cole-Parmer can usually solve any questions you might have about an unknown pump.
Ideally for calcium reactors you are looking for the pump to be 6-600rpm. IME for most peoples applications we will be using LS17 tubing. It has a 1/4" ID and a 3/8"OD. Masterflex tubing comes in two types, precision and performance. Both will work as long as you have the proper head but we will focus on precision tubing since the tubing is more readily available.
**You have to match the tubing to the head or the unit will malfunction.
**
Pump heads vary. Standard load, easy load, quick load... ect ect. I prefer easy load heads but as long as you have the right size tubing any head is fine. They come in Cold Rolled Steel (CRS) and Stainless Steel (SS) variations. Both work but stainless are often in better shape. If you can get stainless go for it, if you can't don't let it keep you from getting a good drive.