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Are you sure you did your testing correctly? Water quality E.coli testing is so sensitive that normally there is no such thing as "not enough".

Although I could imagine the horse-poo was older and and mixed with urine (driving up pH and thus killing bacteria)?

In most cases I worked on we used fresh municipal waste water, but that might not be available in at your site. For animal poo I would look for as fresh as possible one... maybe chicken litter would work? With chicken you can easily keep a steady supply close at hand.

The commercially available E.coli cultures are probably not suitable for this, as they are typically not very environmentally resistant strains. Think of lab-rat vs. street rat.