as mentioned, this spell does not work. real spiritual magick uses energy found in nature. as a force of nature, it cannot go against nature. as humans, we cannot breathe under water, or grow tails, so no spell can physically change us into a mermaid.
but wait! [i hear you say] this spell claims it's '100% working'. well first off people lie, second, people lie a lot more on the internet, and thirdly, there's no real filter for people to post spells on this website, and only one person can delete them, and since he's busy, non-working spells stay for a long while.
granted, your astral self can change [i've been told, never tried] think of your astral self like your spirit [it's not, but it's similar] this version of you can project [or leave] our body and travel to the astral plane. [some people feel this is imaginary, other feel it's real, draw your own conclusion.] anyway, i know people whose astral selves [apparently] aren't the same as what they look like on the physical. meaning they could be a wolf, or they could be a mermaid. this isn't any physical change, and it does take a lot of time and practice in order to project, but that's your option if you want to become a mermaid. [unless you want to go in for surgery, but i wouldn't recommend it]