Deep clean sewage tank

[bazjude2998]

I know you may think this is the wrong forum.I have tried boats planes.I have a 20yr old narrow boat.It has a sewage holding tank 300 litres (125 mm deep)tank has to be pumped out when full we add chemical Blue at the advised amount (apparently too much will kill the good bacteria as well as the bad )Suffering smells which I think is down to a build up over the years of nasties.What chemicals could I use to deep clean the tank.No way of getting into tank to power wash out.Dont won't to risk cutting a access into tank.Any suggestions apart from scuttlein.

[bazjude2998]

Thanks for your reply.The tank is vented but no charcoal filter.The problem has only appeared this last year after 20yrs constant use.In the scheme of things I suppose not bad.I just feel that a deep clean would help.All the obvious have been checked,I am really asking on advice of what would be the best chemical to use.The Agent Orange of the Sh-t tanks.Thanks

[bazjude2998]

Thanks for all replies.In this instant the holding tank is solely sewage,rightly or wrongly all grey water is discharged into the canal system,some interesting chemical suggestions

[bazjude2998]

Thanks Si ,Karcher kit my do the trick but as always nothing easy (that makes life interesting)The tank is L shaped quite large in area but shallow.I suppose we all look for the easy fix in this case it would be great to fill the tank with chemical/ water mix,leave for a couple of weeks drain down,question is what's the best chem to use.