The Blue Water in the Fish Tank

Ivo came to the science room early on Monday. He wanted to feed the class fish. The room was quiet.

He looked at the fish tank. He stopped at the door. The water was bright blue.

Two orange fish swam in it. They looked fine. But the water looked very strange.

"Ivo, you are here early," said Kamal from the door.

"Look at the tank," Ivo said.

Kamal walked over. His mouth opened wide.

"Wow! It was clear on Friday," he said. "Who did this?"

"I don't know," Ivo said. "Maybe someone put paint in it."

"Paint is bad for fish," Kamal said. "But our fish are not sick. They swim and eat."

"So it is not paint," Ivo said. "We need clues."

"Good idea. I look at the floor," Kamal said.

Ivo looked at the table. He saw a small box in the bin. He took it out.

"Kamal, come here," he said. "This box has a picture on it."

"What is it?" Kamal asked.

"It is a toy castle for fish tanks," Ivo said. "The box is empty."

"So someone put a castle in the tank," Kamal said.

"Yes. But I don't see a castle," Ivo said.

They looked into the water. At the back, they saw a small tower. It was very pale blue.

"That is the castle," Kamal said. "It is blue, but the water is much bluer."

"Maybe the colour comes off the castle," Ivo said.

"Let us test it," Kamal said. "A test shows us the answer."

Ivo found two clear cups on the shelf. Kamal filled them with clean water from the tap.

"We need one more castle," Ivo said.

"The box has a spare piece in it," Kamal said. He pulled out a tiny blue flag. "It is the same blue plastic."

"Good. Put the flag in one cup," Ivo said. "Leave the other cup empty."

Kamal put the flag in the first cup. They both watched the water.

"Nothing happens," Kamal said.

"We must wait," Ivo said. "The tank was warm. Maybe this water needs to be warm too."

Kamal ran to the sink. He filled a cup with warm water. He put the flag in it. Ivo looked at the clock.

Five minutes passed. Ten minutes passed.

"Look!" Ivo said. "There is a blue cloud around the flag."

The water in the warm cup turned pale blue. The water in the cold cup stayed clear.

"The warm water pulls the colour out," Kamal said. "That is the answer."

"But who put the castle in the tank?" Ivo asked.

"We have a second clue," Kamal said. "Look at the floor."

On the floor, there were small blue drops. They went from the sink to the tank. A blue hand mark was on the chair too.

"Someone with blue fingers stood on this chair," Ivo said.

"Who has blue fingers on Monday?" Kamal asked.

Just then, the door opened. Joss came in with his bag. He looked at the tank, and his eyes got big.

"Oh no," he said. "It worked too well."

"What did you do?" Ivo asked.

"I bought the castle on Sunday," Joss said. "The fish had no house. I put it in the tank in the evening."

"Look at your hands," Kamal said.

Joss held up his hands. His fingers were bright blue.

"I washed them two times," he said. "The colour does not go away."

"The castle did that to the water, too," Ivo said. "The tank is warm. The blue plastic gives its colour to the water."

"Is it bad for the fish?" Joss asked.

"They look fine," Ivo said. "But we must take the castle out. We must change the water."

"Let me help you," Joss said. "I am sorry."

"It was a nice idea," Kamal said. "But the castle is not a good one."

Ivo took the net from the shelf. Joss took a clean bucket from under the sink. Kamal opened the tank lid.

One orange fish swam up. It looked at them.

"Do not worry," Ivo told the fish. "You get a clear home again."