CLeaning materials for septic tanks

Hi

I have lived here about 20 months and I brought a good supply of toilet cleaner with me but ran out about a month ago.  I have a septic tank and although I can keep it looking cleanish, there is a brown stain beginning to form round the bottom of the loo.  Can anyone please help me.  Can I buy a septic tank safe cleaner here.  I think it needs to remove limescale which is why the staining is starting to form.  I cant find anything suitable in the local supermarkets and dont want to have to buy from England indefinitely.

Does anyone know if vinegar, which I know removes limescale would damage the septic tank biology or not.  Or if I can get anything here.  The nearest and biggest places I know from here are Le Roy Merlin and Carrefore in Reus.  I am 56 K from them so dont want to make the trip for nothing.

Any suggestions would be greatly appreciated.

Thanks

Make sure the air vent for the tank is not blocked the air vent is their to let the gas out of the tank if it is blocked the gas will come up through the waste pipes this will stain the the toilet bowl and will smell as well.

Thanks

I will have to get it checked.