I dug deeper into the available info and found a solution.
The K-30 1% CO2 sensor for $65.00 (Model #SE-0018) appears to always has this undesirable Automatic Baseline Calibration (ABC) enabled, I see no user-configurable way to disable it.
The K-30 1% CO2 Custom for $75.00 (Model #SE-0018-C) has several options, selectable via a PDF form filled out and included with the order. One of these options is to disable ABC. I'd say this option is an absolute necessity, especially if the sensor will be used long-term and closed-loop to control CO2.
These sensors can be knocked out of calibration by mechanical impact, including what's typically seen during shipping. So without ABC, the custom version will need to be manually calibrated at least once upon receipt. Outdoor air would be best for this. As you already found out experimentally, indoor air can be noticeably higher in CO2.
There was no option on the website to order a custom version of the K-30 3% that you just gifted yourself with, though they might be able to provide one if contacted directly.
I have a sneaky suspicion that all these sensors are 100% identical hardware, with options set after manufacture either by UART or JTAG. The higher price on the 3% version is because it's targeted strictly to scientific and industrial markets that will gladly pay the premium without question. The 1% version has broader potential use, for example indoor air quality monitoring, and is priced lower to take better advantage of the broader market. Should this catch on, and a hardcore hacker examine the sensor, it's possible they will figure out how to transform a 1% version to a 3% version; or perhaps even other trade-offs between range and accuracy. Have seen many instances of this happening on the Hack a Day (HAD) website, where deliberately crippled goods are transformed into much higher priced versions by procedures that are simple; at least once they are found. Though HAD's not aquarium oriented, it has featured quite a few aquarium hacks, and a fair proportion of the readers seem to be into aquariums. You might consider submitting this to them at some point.
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