If you've been vaping for some time now you probably leaking tanks are a constant issue. Regardless if they come with pre-built coils or you make your own builds, tanks and RTAs eventually get flooded and start dripping e-liquid through the air holes. For some it's business as usually while for other vapers this can be a major setback, especially if the problem keeps on repeating.
Dont forget to read the manual - find out how it works
Some tanks leak when they are on their side, others when you vape in a certain way or with a certain e-liquid. But what exactly causes this issue and what can we do to prevent it from happening?
Except for some rare cases where manufacturing flaws are at the core of leaky tanks, it's usually user error. Many people ignore the instruction manual of leaflet and think they know better, however it's actually a great idea to read about your device rather than to assume. These problems are usually due to small details we often disregard when replacing a coil head or refilling the tank.
Firmly tighten the coil heads and watch out for faulty o-rings
One main cause of leaking in the case of tanks with pre-built coils is not screwing the atomizer heads tight enough. This prevents the rubber gasket at the base from creating a seal inside the chamber and 
lets gravity take over the e-liquid. Make sure to always tighten your coils and to always tighten all the parts that hold the tank together.
Damaged O-rings are another cause of frequent tank leaking. Made from silicone or rubber these fragile parts can easily break from intense friction and cause issues. It's ideal to check all the O-rings and seals each time you take the tank apart for cleaning or refills. Also lubing these parts with a little bit of e-juice can improve their lifespan on the long run.
Learn how to create negative pressure inside the tank
Some tank models and especially RTAs require careful refilling. This is due to the way they are designed and it's something you shouldn't ignore. If you have constant leaking issues with a device, try closing the airflow control and filling only ¾ of the reservoir with e-liquid. Then, after two turns of the top cap put the tank upside down and continue tightening until it's perfectly sealed. Only then put it upright, open the airflow control and take a few puffs. This is a trick to create negative pressure inside the chamber.
Cotton placement is the most important aspect
However, the most common source of leaking in the case of rebuildable atomizers is the poor use of cotton. As a rule of the thumb, your wick has to be at least twice the diameter of the coil. With some atomizers that lack a juice flow control, you can stack even more cotton. The ideal situation is when you can gently pull the wicking material and it slides through the coil without bending it. Practice makes perfect in this case, as long as you're careful not to overdo it. Too much cotton and you're looking at constant dry hits.
Last but not least, the thickness of your e-liquid plays an important role in the matter. Juices with 70% VG or more are less likely to leak than thinner alternatives with 50-60%VG, given the same atomizer.
Read more about the type of e-liquids to use with certain tanks.
