Flush tank additives

I recently started using biological liquitabs to add to our waste tank. I found that they worked well with little difference between them and the expensive stuff you can buy in caravan dealers.

I was wondering has anyone any experience with additives to the flush tank?  I would have thought that any suitable wetting agent would do?  Has anyone tried washing up liquid...or am I being naïve?  I tried car screen wash and that seems to work very well but it's not cheap and a lot more expensive than washing up liquid!

Any advice?

Many thanks,

Bob D.