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But seriously, they are probably hiding under/inside something. Mine go missing for days on end and usually are hidden deep within one of the fake tree roots. They can get themselves in very tight spaces.
Have you physically looked inside and under everything in the tank?
If it worries you enough to make two threads in three hours in the wrong section, you can pull the decor out of the tank. If you still can't find them it's probably a user error.