Fiber is food sold in Bikini Bottom at the Discount Grocery Mart. It can be seen in the episode Penny Foolish. Mr. Krabs buys this. It was sold by the the Saleslady. It was also shown in Mermaidman vs SpongeBob to save them from serving Plankton for a long time.
Saleslady: Price check on industrial size econo pack of raw fiber.
Mr. Krabs: Ah, it goes in like steel wool, but comes out like an angel from heaven.
Saleslady: Please, tell me more.
Mr. Krabs: Ooh, well, this one time I...
Saleslady: Not really!