Sand can change a growing mix, but grain size matters. Check drainage and the rest of the ingredients before assuming any sand will improve compacted potting soil.
Decide what the mix must do
A growing medium needs to hold roots, moisture and air in a balance suited to the plant. Adding sand does not automatically improve every mix. Particle size, proportions and the other ingredients determine how the finished medium behaves.
Avoid unsuitable material
Fine sand can fill spaces between larger particles and make some mixtures denser. Use horticultural material appropriate to the intended mix rather than assuming builder's or beach sand is interchangeable. Salts, contamination and inconsistent texture can create problems for container roots.
Test the complete container
Combine ingredients evenly and assess water movement and drying in a draining pot. A layer of sand at the bottom is not a substitute for an outlet. The medium above it still controls most of the root environment.
Match the plant and conditions
A moisture-loving foliage plant and a small cactus need different balances, and pot size also affects drying. Change a healthy plant's medium for a clear reason. After repotting, inspect root-zone moisture rather than continuing the old watering interval automatically.