A thing to remember here is that these posts are strictly meant for the particular planet position. Like, today we are looking at Mercury in Pisces, so the description given here may change for you if your Mercury is although in the sign of Pisces but with conjunction with or aspected by other planets.
So, lets see what Mercury can do in Pisces without any conjunction or aspect from any other planet. 1st lets see what these 2 things represent --
Mercury represents communication skills, marketing, calculative ability, younger siblings, hobbies, skills with hands, intelligence, quick decision making, logical thinking, youthfulness etc.
As Pisces is the 12th sign of zodiac belt, it represents the same energy and things which are represented by 12th house such as Isolated Places, Spirituality, Imagination, Other Dimension or Other World etc. Besides these things, it also represents your Hidden Talents, Ability to work in background, Spiritual Self etc. Pisces is consisted of last 2 and half nakshatra, i.e. Purva-Bhadrapada, Uttara-Bhadrapada & Revati. Pisces' lord is Jupiter.
So when Mercury is in Pisces, it gets debilitated at 15 degrees. Why? Mercury is intellect of a person and Pisces is sign of imagination and dreams. So, obviously intellect has no role to play in the imaginative and dream world of a person. Hence, Mercury is debilitated.
But there is much more than this into interpretation of this planetary placement. This placement doesn't mean that a person's intelligence is not working or they are dumb. It simply means their intellect is more active in dream world than in real world. They are highly imaginative people and the field where this imagination really works well is writing. It is one of the biggest amusing thing that a debilitated Mercury (communication/writing) makes the most of best selling authors of the world. Their imagination level is so high that they get extremely new ideas for writing. Otherwise also, in normal walk of life either these people would speak very less or they speak of such high morale values that a poor normal human being would be unable to relate to it. Best Ex - Sachin Tendulkar. Give them a normal Mercurial work of accounting/calculating etc and they will mess it up. But give them any work related with imagination, they will excel. As Pisces is also sign of Spirituality and Mercury remains the planet of collecting information and communication. These people love to read about Spiritual stuff and communicate about it. A great placement for any imagination related work.
As always, Mercury's placement in different nakshatra will change this scenario as per the position of Nakshatra Lord. If Mercury is in Purva-Bhadrapada Nakshatra, its lord Jupiter's position would impact Mercury. If Mercury is in Uttara-Bhadrapada Nakshatra, its lord Saturn's position would impact Mercury. Likewise, if Mercury is in last nakshatra Revati, then it is best for Mercury as it is in its own nakshatra.
Overall, Mercury will be impacted most by the condition and position of Jupiter in Horoscope as Jupiter is lord of Pisces.
Vishal S Saxena - Astrologer