Can you put cardboard box material in a tank with fish?

No, cardboard boxes should not be put into a tank with fish. Cardboard is composed of cellulose fibers which are biodegradable, and may release harmful chemicals into the water as they decompose. These chemicals can be toxic to fish and can cause health problems such as skin irritation, fin damage, and stunted growth. Additionally, the decomposing cardboard can contribute to an increase in organic matter in the tank, which can lead to poor water quality and oxygen depletion.