We use bleach, Domestos or Morrison's own (both state septic tank safe) and vinegar/bicarb which is fabulous for unblocking shower drains! Monthly muckmunchers too.
Our tank was fitted early 80's I think (mushroom style) but we had to bite the bullet and have new pipes last year. Chap actually found the old pipes (twice) whilst digging new route with laser. Mum said she remembers the old clay pipes being put in and they didn't use lasers! Field also slants several directions.
Mine's the same - serves three houses, all built about 30 years ago, emptied by Scottish Water once a year. Works fine.
By the way, all Septic tanks need to be registered with DEFRA now
Going slightly off subject......
Only the biggies. Mains sewerage finishes next door and our property and several others have our own tanks, we don't share. The reason mum knows that our septic tank was only put in, in the 80's (in the property we live in) is because she was born next door (we're a semi) and when she (with us) decided to come back in the 70's it was extended for a family of mum and 4 girls and INDOOR PLUMBING was installed!! 1980's winters with thick snow, lead range in kitchen to keep us warm and ice on inside of the windows. Gas CH wasn't installed until the mid 90's with the double glazing when she had some money! That septic tank was brick and sister had new tank/pipes installed 2020!
An old aunt lived in our current house until the 80's, went into hospital and council wouldn't let her come home until her home was deemed "safe", my grandad (her brother, no financial help back then) had to pay for indoor plumbing, bathrooms (down and up) fire place had to be bricked up, gas fire installed and gas CH fitted (he had to pay for the connection!) Then she died in hospital!
So mum had a door opened between the 2 homes and we spread into this side and grandad came to live with us.
One reason mum gets angry with governments giving money to people to improve their homes, she couldn't get any and worked and worked and worked. Even now, we had to pay ourselves to replace the pipes, our double glazing needs replacing as some of the seals have gone and you can feel draught, but if you don't have DG you can get it free but there is NO help for those that have aged DG. She was entitled to the WarmHome scheme 12 yrs ago and was able to receive a NEW gas boiler on condition the MF stove was ripped out and replaced with a gas fire. We scraped together the money for a new boiler and the MF stove is currently chuckling keeping us warm.