This is my first post on here, so please move if needed. The post might be a bit lengthy but I thought it's better to start from the beginning to hopefully get some advice from anyone who's experienced with cats. I have had Barbara (blue British) for a year with 0 health issues. She ate normally, was brought up on royal canin kitten then weaned off to whiskas pouches and dry food which she loves. About 10 weeks ago we bought Stan (persian chinchilla kitten). Stan came to us on iAMS wet food and his poop was never fully solid with it, so I decided to try and wean him off it and try whiskas kitten wet food. Stan had then developed full on runny diarrhoea. After 2-3 days of full on runny poo we took him to the vet. The vet advised us that he's most likely had a bad reaction from whiskas and to just give him boiled chicken and rice to 'reset his tummy'. Stan had a week of rice and chicken and not an ounce of anything else and continued having diarrhoea every day. We then went back to the vet who prescribed Stan worming medication as well as an antibiotic. Stan had 5 days worth of both of these whilst now being fed chicken only (vet also said it might be wheat or grain intolerance so to cut out rice) and still continued having daily diarrhoea. We were running out of options and the next step was to send off Stan's poop to a lab to get it examined, however at the same visit the vet suggested some probiotic and Royal Canin Gastro health sachets, to try settle his diarrhoea a little bit whilst he undergoes the investigations. As soon as we started giving Stan those sachets alongside the probiotic his diarrhoea settled. We thought we found the miracle cure. We continued giving Stan one blob of probiotic every morning and gastro sachets (1 sachet a day alongside the same type of dry food). The vet said 'this is working well for him, he might have an intolerance or an underdeveloped gut so keep him on the gastro food). So we did as we were told. After about 4 weeks of just gastro food alongside the probiotic, I started introducing whiskas again, and he seemed to be perfectly fine when i substituted his dry food for whiskas dry food. I then started introducing whiskas wet food and giving him 50%gastro / 50% whiskas sachets for every meal alongside the probiotic which again didn't cause major poop issues. I then stopped the probiotic all together for a couple of days and we are now on day3 of constant diarrhoea every time hw goes to the toilet. Baring in mind stan is VERY fluffy and VERY white, he has to have a basin-bath every time he uses the litter tray. The second issue is with Barbara my first kitty. For about four weeks now she also has intermittent diarrhoea. Some poops are completely fine and solid and normal for her, then every 2-3 days she has a full on liquid, high in volume diarrhoea. As I said, before Stan came along, she has never suffered from anything, ate what she wanted, we even fed her some of our dinner before, if she kept scrounging under the table, and it seems to be a strange coincidence that she now has diarrhoea bouts. Her food hasn't been changed at all during this and she doesn't eat Stan's special food. I should add that both cats are indoors cats and don't go outside. They share a big litter tray. They have separate food and water bowls. Both have monthly advocate (flee) treatment and 3-monthly drontal (worming) treatment (Stan had his first one about 4 weeks ago). Im just a little bit lost with it all, as, if it was of parasitic nature, then Stan was treated by the vet with a 5 day course and Drontal later on. If it was a viral cause, it wouldn't have lasted so long.They don't have any other symptoms apart from diarrhoea which would exclude bacterial infections. (Im a medical professional so I try and think like a vet here...) This would leave us with simple food intolerance, but I would love to know if any of you would be kind enough to share your thoughts on the matter. Many thanks in advance, any comments welcome!