Decans have some significance. They're one of the weakest forms of dignities, think one of the latest layers going from Domiciles->Exaltations->Triplicity-> Bound -> Then finally decans. You'll end up with a lot of rulers if you go by that logic, but if you can find to make it work for you then all the power to you.
A lot of its knowledge/application has been lost when it comes to decans. It fell out of favor and usage as time went on, and various authors have something to say about it. It being just a very minor dignity, but some authors also talk about their power and strength.
They come from ancient Egypt, where they grouped 36 star systems in their astronomy system. These star systems rose south of the ecliptic every 10 days over the course of the year. Each rising decanate marked the beginning of the hour or "decanal." They were used as a sort of clock system. https://en.wikipedia.org/wiki/Decan
They were also inscribed in coffins to help the dead with prayers during their journey to the underworld.
Later on, these segments came to be associated with Gods, and represented various things such as the illnesses they bring and the cures of such illnesses. In the earliest texts, there were instructions on creating an amulet for a person with the decan rising at the time of birth. The amulet would protect them from the illnesses associated with the decan. These amulets would have the Egyptian gods, and instructions on what to do, e.g. avoiding certain foods.
Firmicus Maternus writes, "Each sign is divided into three parts, and each part has one decan, so that in each sign there are three decans, each having ten degrees out of the thirty and over these ten degrees it exercises its power and control. They have infinite power and freedom in indicating the fates of men."
He then goes on to write that a planet, even if it's unfamiliar to this place, is like in its own sign, meaning it can fulfill the significations of the planet just fine. He writes that the knowledge of decans have been kept secret and obscured by the Egyptian priests to keep the knowledge sacred.
Rhetorius the Egyptian uses them in his interpretations. In his compendium, he details the effects of the Sun in various decans.
For example, since the Sun is the soul, the Sun in the 1st decan of Aries will be "manly, irascible, delighting in war, fond of weapons..." in the 2nd of the Sun "magnanimous, loving of fame, and fond of honor and no longer delighting in war" in the 3rd of Venus " a womanly soul, in spirit and appearance."
Teucer of Bablylon uses decans to help describe the person, if the Rising sign or the Moon were contained in that sign. The Rising and the Moon were considered to be the rulers of the body, so they used these more often in personality readings.
For example, and I'm paraphrasing, if the person had the Rising or the Moon in the 1st decan of Aries, it will show those who have a flushed face, long nosed, black eyed, slim, shapely, skinny legged, pleasant voice, energetic, ruling, leaving their home lands and passing through much land and sea...etc. In the 2nd... rich, beset by sorrows and losses in the recovery of possessions. 3rd decan... many toils, being involved in dangers or imprisonment, and due to hatreds it will make them leave their own country in youth and returning late and being separated from it again. they have no children.
As for Aquarius/3rd decan (Rising or Moon).... Teucer writes "And in the 3rd decan, those who are kingly, being side by side with kings, and experiencing many dangers or wounded from waters."
These all have a variety of descriptions. They are also used in magic throughout the ages.