We can't get rid of our horses fecal water, advice?

Hi all!

Me and a friend have an Oldenburger horse, his name is Diablo! We've had him for 3,5 years and he is about 17 years old. He is the sweetest gelding and we love him a lot.

For the last 6 months, he has had fecal water. His poops are normal, but we can't seem to stop the fecal water. We don't know where it came from as we changed nothing in his care, but we've been trying to fix it. He lives in a paddock paradise with unlimited access to hay and grass, and is in good body condition. He is regulary dewormed and tests show he has no sand in his poops. Together with our vet, we tried the following things: grass management, different hay and feed, adding extra fiber in his feed, a psyllium fiber treatment.

Unfortunately nothing has stopped the fecal water and our vet is running out of ideas. Further testing will cost a lot of money, so we would like to see if there are any people here that have an idea as to where the problem might be coming from or what else we could still try before further tests.

We look forward to your insights and advice! Thanks in advance!