Hugo Holiday Blocks keep your fish fed and happy for up to 14 days. The calcium carbonate block dissolves slowly over two weeks, releasing food particles into the water. As a guide, one block placed in a 45-litre aquarium should be sufficient to supplement the diet of 10-15 average aquarium fish for approximately two weeks. Hugo Holiday Blocks are not suitable for marine aquariums and are recommended for tank temperatures between 20-25 degrees.