Feeding blocks to feed fish for up to 3 days. Ideal for weekend outings.      How do they work? Block feeding involves placing a block of compressed food in the aquarium, which slowly dissolves and releases food for the fish over several days. This way of feeding is useful when you are going to be away for a while and cannot feed the fish daily. However, there are some things to consider before using this method, such as the size of the aquarium, the number and type of fish, water quality and temperature. Not all blocks are suitable for all fish, and some can upset the chemical balance of the water or cause overfeeding.