Chemical cleaning of sewage holding tank

[bazjude2998]

Hi.we own a 18yr old narrow boat which has a sewage holding tank which I suspect needs a deep cleanse.I,m thinking through the winter having a pump out and rinse,filling with clean water and chemical Blue (formaldehyde based )and leaving for several weeks.Good or bad idea?Couple of questions,the base of the tank is the base plate of the boat all steel,would a consentrate of the chemical have any adverse effect on the steel?The tanks capacity is approx 300 litres what ratio of mix would you recommend?Finally is there any alternative suitable chemicals available.Thanks in anticipation.Regards Baz