My problem with the tadpole example is that you have two skill checks for the same skill right in a row. I don't personally think that is a fair approach for the players (who tend to enjoy succeeding skill checks). It just seems like a veiled attempt to make them fail.
Yep, that's the wrong way to do it. If you want them to fail the check, make the DC higher in the first place.
If you need multiple checks have them make sense and be diversified. For instance in that scenario, if you pass the skill check to kill the tadpole but want to add some drama - make it slither away faster than expected and require a dexterity check from ANYONE in your party to catch it or a perception/investigation check to see where it went.
And this as well.
