Mars twice over
The Chaldean order steps through Mars, Sun, Venus, Mercury, Moon, Saturn and Jupiter in a loop that begins at zero degrees of Aries and runs through all thirty six faces without resetting. Seven does not divide evenly into thirty six, so the sequence closes on the planet it opened with, and the last face of Pisces takes Mars.
The triplicity scheme reaches the same answer along a different path. It hands a sign's faces to the signs of its own element in order, and a Pisces sequence runs Pisces, Cancer, Scorpio. This face therefore belongs to Scorpio, which traditional astrology also gives to Mars.
Agreement between the schemes is uncommon enough to be worth noting. Across most of the thirty six faces they name different planets, which is why a decan ruler quoted without its scheme is unverifiable.
What the pairing is traditionally read as
Water under Mars is read as directed rather than diffuse. Where the earlier faces of Pisces are described as absorbing, this one is usually given as the part of the sign that acts, and older texts treat it as the least yielding stretch of an otherwise yielding sign.
That is a reading of a band of longitude and it is offered as one. It is not a trait the calendar confers, and nothing here forecasts anything about a life. The tradition attaches meanings to positions, and this page reports the attachment while keeping it labelled.
It is also worth separating the planet from its reputation. Mars in this scheme marks a face rather than sitting in one, so nothing here says a planet is placed anywhere in your chart. It says which ten degree band the Sun occupies and which planet the tradition attaches to that band.
Where the Sun actually is
Taking noon positions from 1930 through 2035, the Sun on 12 March stands between about 21.0 and 22.4 degrees of Pisces. That entire range sits above the twenty degree decan boundary and well below the thirty degree end of the sign, so neither edge is in play.
The band is narrow because the leap day keeps the calendar tied to the orbit. Four ordinary years let the date slip about eighteen hours late against the Sun, and February the twenty ninth pulls it back. Over a century the drift never accumulates into more than a degree and a half.
That stability is the useful part. It means a decan reading for this date holds for anyone born on it in any year of the last century, which is rarely true of dates sitting on a boundary.
The hour still decides most of the chart
The sun sign on this date is not open to argument, since the equinox is more than a week off. Almost everything else is. The Moon covers about thirteen degrees a day, so a birth in the morning and one at night can put it in different signs entirely.
The rising sign depends only on the local clock time and the latitude of the birthplace, and any of the twelve can be on the horizon on 12 March. The widget above computes your solar degree from your own year and place and reports how far you were born from the nearest sign boundary.
Houses shift with the hour as well. The same Sun at twenty two degrees of Pisces can fall in any of the twelve houses depending on what was rising, and house placement is usually read as mattering more than the decan does. That is another reason the date on its own can only take you so far.
Questions people ask about this
Who rules the decan for a 12 March birthday?
Mars, under both the Chaldean order and the triplicity scheme. The Chaldean sequence closes on Mars at the end of Pisces, and triplicity gets there through Scorpio, which is also a Mars sign in traditional practice.
What degree of Pisces is 12 March?
About twenty one to twenty two and a half at noon, depending on the year. The widget above gives a single figure once it has your year, hour and birthplace.
Is a 12 March birth ever Aries?
No. The Sun does not reach zero degrees of Aries until around 20 March, so this date falls inside Pisces in every year on record.
Does the Mars rulership make this a harsh birthday?
That is not how the technique works, and the site does not read positions as lucky or unlucky. Mars is described as directing rather than softening, and how much weight to give that is a judgement.
How close is this date to a decan boundary?
Not close. The nearest is at twenty degrees, roughly a day and a half behind, and the next is the end of the sign more than a week ahead.
What varies between two people born on 12 March?
Nearly everything except the Sun. The Moon may be in different signs, the Ascendant almost certainly is, and the houses that each planet falls into shift with the hour and the place.