I buy pure KCL online, and add ~17g/ 100ml, which gives about a 3M solution which is what Apera uses according to their website. The homemade stuff has no preservative in it, so it eventually seems to grow stuff. Since it is so cheap to make, I just change it regularly. It stays cleaner longer if you rinse the probe with RO before replacing in the storage cap.
The biggie with these pens is never let them dry out. If they are not taking readings, they need to be stored in solution. Some tolerate this abuse more than others, but drying them out harms them all sooner or later, and the cheaper the pen the quicker they fail.