Many thanks for your question, I'd be happy to assist. You will need to clean the surface from old paint and loose material. This can be done with an angle grinder and a masonry wheel. Once you have a clean surface you can use the product Gripset Betta 1kg Water Plug over the crack to stop the leak. The product is ideal for use in wet areas and cracks in water tanks like the one you have shown.
Please let me know if you need further advice or had questions.