If the o-ring was rigid, it was definitely old or dried out. There are no hard o-rings that come stock on this filter. The only time I have had to replace o-rings or seals is when I pinched them during assembly or when I bought a used one that had been abused or was missing them.
Filter is not even a year old, maybe it was my fault for not putting grease or lube on it when I first got the filter. No instructions that tells you to apply neither, they should. Oh well, problem was fixed by overkill but no more tiny bubbles in tank. Water is crystal clear