Hello,
Yes, you are not limited to Tesco brand cat litter. In fact you are not limited to cat litter. The product we know as cat or kitty litter is simply clay baked at a temperature on the order of 1200 degrees Fahrenheit. The clay is then put in a grinder to achieve a specific grain size. This particular technique of firing produces the raw product called "Calcined Clay". This process generates small air pockets in the clay granules allowing the granules to be extra absorbent. This, combined with the fact that clay has good liquid retention is why it works well to absorb cat urine.
You can find this type of clay in almost any product that is sold to absorb liquid spills on solid surfaces, especially hazardous liquid spills. This type of clay product is also used as terrestrial soil enhancer as it retains water without being messy. Bonsai growers use calcined clay products to help ensure that the plants don't dry out.
In cat litter, perfumes are often used for obvious reasons ,so if you are buying cat litter then ensure that you avoid the perfumed brands as the chemicals being used for odor control may have toxicity for fish. Check the ingredients to make sure it states "calcined clay" and non-perfumed - or similar expressions. If you have access to a bonsai nursery, check their prices (as these are sometimes overpriced). Check hardware shops for liquid spill cleanup products.
For our purposes, we really don't care all that much about the water absorbency or the porosity because although this is excellent for land plants, these are less meaningful parameters for aquarium aquatic sediment. What we care about is that it is made of clay and that it won't turn to mush or cloud the water by being too soft. Avoid products with toxic additives, experiment with different products and compare their performances.
Cheers,
