Even though the vast majority of tarantulas are native to areas with high humidity, keeping your terrarium humid can be a problem.
Terrains occupied by most tarantula species have a humidity percentage of 60-90% saturation during certain times of the year (usually half way through the year), but unlike your terrarium, they have good ventilation, thus avoiding the proliferation of unwelcome creatures. Breaking some established rules, I recommend that you keep your terrarium semi-dry. Why? What does keeping moisture in your terrarium mean? This could make it a great culture broth for any degradating little organisms, such as mites and fungi, that in turn attract other predators, and soon you could have an entire zoo in your container, and a very unhappy tarantula. What happens if you have a semi-dry container? Nothing! I have proven it thousands of times. Nothing at all happens, just as long as you have a wide and deep water dish with plenty of water, as we have mentioned before. In most cases, this, and the small amount of moisture in the substrate, will provide enough moisture for your terrarium, so you won’t have to bother moistening your tank.
The only times where I would recommend humidifying your terrarium would be, first, when you just got your tarantula in order to make it easier for it to build its burrow; and, second, when you see your tarantula spending a lot of time standing on top of its water dish seeking moisture.
How do we keep the substrate moist? Never spray it with spray bottles. This can really bother your tarantula. Just take a glass of water and pour it over the terrarium walls, just as it is shown in the video. It doesn’t matter if a puddle forms at first, the water will be uniformly distributed by capillarity in a few hours. See video showing method.
If you get carried away with the moisture, don’t worry, in time it will evaporate.
If you should have an infestation of mites due to the humidity, the solution is very simple, just stop moistening the substrate and you will see how they dive into the water, immediately drowning. This will get rid of them all, but make sure you change the water after this massacre, of course.
If you have a tree-dwelling tarantula and you want to keep its humidity level constant, as I have explained in the section on substrates, you should use vermiculite. Since this is an inorganic matter, it won’t cause infestation problems.
It is very important to always keep your containers free of food residue, since this is the main cause of infestations. Once you have fed your tarantula, on the following day, with no exception, remove all bits of food she probably left in a corner.