Ever since I've heard stories of people in the U.S. who (almost) emptied their accounts, I've feared of one day doing that too. I always make sure to have at least 100 EUR in each of my accounts. It goes like this: Someone takes everything but 5 USD out of their account. The bank says it's too low and charges them 20 USD. Their balance is now at -15 USD. The bank charges overdraft fees. After some time, their balance is at -16 USD. It's still too low. The bank charges them another 20 USD for having too low of a balance. Their balance is now at -36 USD. The bank charges overdraft fees again. Their balance is now at -38 USD. Etc. Even though they had a positive balance before this fee cascade started.
Are banks in the EU allowed to charge fees for having too low of a balance? I'm not asking about overdraft fees like 20% annually for how much below zero your balance is. I'm asking about either non-relative charges (like 20 EUR for having a balance below 100 EUR or below -1000 EUR) or charges for a balance that's low but not below zero (like 20 EUR if you hit zero or 20% of how much you are below 100 EUR).
If there is no EU-wide regulation, I'd take comfort in there being a Germany-wide law regarding this because I'm German and for the most part use German banks (one in Luxembourg).