Personally I don't like to say about tanking generally, it's rare thing. If you ever were engaged in any sport activity seriously and for long time, most probably you know it's impossible to play every match on the same level and with same attitude to rival...
First, i agree that it is impossible to play every match on the same level and with the same passion and motivation. Everyone who played club tennis knows this. You have a good day, you have a bad day and you have very good and very bad days.
But we have to talk about two possible tank possibilities.
1) Tanking doubles (and singles matches)
2) Tanking Qualies because you have a LL spot
Second one is not that common and a bit rare, yes. But still there. Look what in Bangkok happened. All seeded players won final round of Qualies although they had 2 LL spots because of missed flights and some other stuff.
But 1) is definitely a problem. You can find a lot of guys who losing doubles middle/end of the week and playing CH/ATP Qualies at the weekend. Of course this is not a blatant tank, maybe they give everything in their doubles match but there are a lot of examples for blatant tanks.
I can remember Enrique Lopez-Perez losing a doubles match in Morocco this year (it was a challenger) to play Challenger Qualies at the weekend and yes it was a tank. He served with 100kmh and hit almost every shot out of the court. Personally, i would give a fine for this. Why signing in and then tanking in the first round? This is a joke. But Lopez-Perez is a bad example. The guy is a clown. He tanked a ITF qf or sf this year to play Challenger Qualies. What a mug. He should receive a ban or a fine for this because he did it several times this year, doubles, singles, especially doubles.
I would give more WCs for Challenger MDD. More talented, young local players who try everything to win a couple of games or a match. They are motivated and they won't tank.