Chain loosening over time (Fixie)
Is it correct that with fixie you shouldn't keep the chain too tight because it can ruin the drivetrain?


That is mostly correct. Given that the circularity of the chainring, bearings and rear sprocket are never ideal, there will be a position through a full crank's rotation when the chain is the most tight, and a position where it is the most loose. It is recommended to find the tightest spot and tune the chain tension to be just right in it. Then, just in case, you can find the loosest spot and make sure the chain won't hop off the teeth if shaken in the second position.

Overtightening a chain would not give performance benefits while simultaneously it would start pressing both chain and chainring excessively, not benefiting their wear life.


  Is it normal that I have to tighten the chain probably at least once a month because it gets too loose with casual riding, no more than a 100 miles per week?


If it is not the rear wheel axle sliding forward because of a low torque on its axles or because of bad nut design/quality, then it is the chain. You would have noticed the axle sliding because the rear wheel would become misaligned in the rear dropouts and the tire would start rubbing one of the rear chainstays.

It is possible that a chain of poor quality would wear faster than desired, effectively "stretching" itself and thus becoming loose on the drivetrain's teeth over time. I've read reports that so called half-link chains used in BMX are quite prone to it. Try using a better, possibly more expensive chain.

Again, if the chain seems "loose" to your taste but you cannot throw it off the drivetrain's teeth by hand, it is unlikely it will come off during riding. In that case, let it be for a while.

And again, do not forget about regular maintenance of the chain, that is, replacing it after it has worn and become longer by 0.75% of its original length.