Fantasy book with young female protagonist, magic, matter manipulation, tasteless chocolate sandwiches and room with aquarium walls

Question

I'm trying to identify a book I read in the mid-2000's in English, which I'm fairly sure was the first in a trilogy. I remember very little of the overall story-arc, but a few scenes stand out:

• The main character was a young girl who somehow ended up in a world that wasn't her own
• When she first came to this new world she was in a massive room that at one point had enormous aquariums for walls, filled with colourful fish
• In this world you could create matter somehow, or reshape things, and she was told if she was hungry to visualise food and it would appear - she visualised a chocolate sandwich, and while it looked perfect, it tasted like nothing because she hadn't thought of the taste when she called it into being
• There's a witch in the story, who has spiders in/around her mouth? And at one point the girl is terrified she's turning into the witch (her teeth are falling out and she's finding spiders in her mouth), but ultimately she realises it was her fear of turning into the witch that was causing this, as she was unconsciously transforming herself due to her fear, which only exacerbated the issue.

I've done a bunch of key-word searches in google but am finding it very difficult to track down.

Answer 1

While I can't swear to it (not having read the stories), I think this might be the The Doomspell Trilogy. Summary of the first book, The Doomspell:

In The Doomspell, Rachel and, Eric are sucked into a freezing world called Itrea, by a witch called Dragwena. They meet a nice old man named Morpeth, who is over 500 years old and, like Rachel and Eric, was removed from Earth to someone by Dragwena. Rachel is fooled into believing that she is becoming a witch by Dragwena, and her Magic, trying to be helpful, tries to make her a witch. During the transformation, Dragwena lets Rachel inside her mind, giving Rachel access to the spells there. As Rachel's skills develop, she finds that she is more than a match for Dragwena. With the help of the Wizard Larpskendya, she kills Dragwena, and Larpskendya removes the spells making it snow. Morpeth then returns to Earth with Rachel and Eric.

TVTropes mentions tooth-cleaning spiders as part of being a High Witch:

The High Witches were bad enough, with their red skin, gigantic flappy noses, teeth-cleaning spiders and four moving and interchanging jaws, but holy shit the Griddas. Gigantic shielded eyes, brown fur, orange skin, five jaws (one on the back of their head) and claws.

And a quote about the tasteless chocolate sandwich from Google Books:

Rachel gingerly picked up one of the sandwiches and took a small bite.
"Yeuch!" She threw it on the table. "It tastes disgusting!"
Morpeth laughed out loud, big wrinkles creasing around his cheeks and mouth.
"It's not funny," Rachel said.
"Ah, but you forgot something else!"
"Did I? No, I'm sure-"
"You forgot to imagine what the sandwiches would taste like!"
"Oh." Rachel realized he was right. She quickly pictured the taste of mingled bread and chocolate and nibbled the edge. This time it was perfect.

My search terms (4th or 5th try) were book series witch spiders teeth.