Ensure the tablets are not ...

Ensure the tablets are not contacting the iron steel when dissolving - they should be added to a floating basket.  If chlorine has only just be added then there may have been a severe level of biological corrosion already.  Look up sulphate reducing bacteria they will cause holes & pits in the metal which will significantly increase the corrosion of the metal. Chlorine is not that effective on established micro corrosion since these bacteria form tubercles that protect them from the chorine. Water corrosion is related to the LSI (Langlier Saturation Index) this is related to pH, calcium hardness & alkalinity of the water.  In order for the water to stop corroding the metal surfaces this LSI needs to be between -0.5 & 0.5.  If you e-mail me I will send you a copy of the calculation and you can check where your water is situated but you will need to test the pH, temperature, alkalinity and calcium hardness of the water.