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Concrete potable rain water collection tanks leaking. Need sealing / skimming

Two of our 25,000 L concrete water tanks have started leaking from small cracks that have developed, its happened before and they sealed themselves back up ove the last 10 years, however I believe we need to have them properly skimmed on the inside to get them to seal up this time, there are three main leaks each about over 1 meter length where the seams are (They appear to be made from three rings stacked on top of each other. Can you please advise / quote how much to seal the tanks up and stop the leaks. I have drained both tanks down to the last maybe 10" of water which I can dump the day before you arrive if needed.