Cole-Parmer. though not cheap but you don't want cheap. Ebay is a good place to look for rebuilt ones at a decent price.
but when the tank and investment are large it is a small price to pay.
The max it would ever need to do is 20 GPD. Thats the most my system has ever evaporated in a much drier climate then I am in now.
Did someone say TKERacer619? On the scene. Let me do some digging for you. What are you comfortable spending?
The only question I have for you is what does this mean? Why are you associating evaporation with a calcium reactor? Is this for a kalkwasser reactor top off or a reactor with media and a co2 bottle?
What sort of demand are you getting now? Do you have a pump already feeding the reactor? If so can you measure your current effluent rate for me? Also is the tank grown out or is it a bunch of frags? For that reactor I am almost sure you are going to want to stack a second or maybe even a 3rd pump head onto the drive.
Good drive/pump, looks like it is in great condition. Ebay wouldn't let me look at the ended auction like normal but I was able to get a full size pic. Looks pretty good. If you want a refurb digital pump I might be able to point you in the right direction but tbh I wouldn't worry about it for now. Enjoy the pump you have. You really shouldn't need a backup.
I would go with L/S 18 on a single head and see if that gets you where you want to be.
Here is a chart that will show you the tubing inside diameters. CP considers it proprietary so you won't find an official listing.
http://masterflexpumps.com/L-S-pt.php
I think you are going to end up stacking a second head on to reduce RPM. This is about tube longevity and a little about noise but mostly tube life. Try it out, the pump will absolutely supply enough flow with 1 head. If you find you want to add a second head you would just cut rpm in half.
You want to use Tygon tubing. I use the phar-med tygon from us plastics but I don't think they have one that corresponds to the LS 18 spec.
I use 2 heads with LS 17 tubing. Pump spins at 25rpm. Never had a tube break but they seem to wear out around 3 months. With 1 head it would last around 1.5 months. I think you can stack up to 4 heads onto the pump you have should you need it.