Ask the Veterinarian/diaria
I have two cats with diaria. One is a 10 year old main coon who has had it all her live. She had several tests in Holland but nothing came out of it. She also doesn't seem to have a problem with it and is very healthy and alive. Still plays the whole day. The cat came from the street and is about 4 months old. she has been with me for just 2 days now but lived the last two weeks at my bosses house wher she got anything to eat what the boss ate. everything with onions, peppers and spyces.
I live now on a small isle in greece and we only have a vet in the weekend. The pharmacy here has only one thing for diaria for animals and those are big sachets with sulfamycine. The man told me they are actually for lambs and other small farmanimals. Can i give this to my cats in the water? and is it a problem for my two other cats? Or do you have another solution without medication? One problem. Lady the old one, doesn't eat anything but dryfood. Not even milk or cheese or chicken or fish.
Hope to hear from you soon.
It could be that you have two totally different reasons for having diarrhoea. A younger cat is certainly more likely to have infectious causes of diarrhoea, such as Giardia, Coccidia, viral or bacterial causes. With an older cat with chronic diarrhoea it is less likely to be infectious if she has had it forever.
I think adding antibiotics to water is often pretty ineffective, most cats wouldn't drink the water and getting an effective dose into them would be pretty hard. Is you youngest cat up to date with worming? I would use a broad spectrum wormer like fenbendazole or Drontal if not.
Generally with diarrhoea I like to firstly feed a bland diet of just cooked chicken (with no fat) and rice, feeding small meals frequently for 5 days, then gradual reintroduction of the normal diet.
With your older cat it does sound as though it could be a food intolerance or allergy or possibly inflammatory bowel disease given the chronicity. One thing you could try is feeding a novel protein diet. Basically don't feed any regular cat food, just feed her an unusual meat like venison, rabbit, kangaroo, duck (the idea is you are doing an elimination diet). It might be a little tricky to do if Lady loves dry food though. If you have access to Hills Z/D dry food (perhaps order online?) you could also just feed this for 8 weeks instead.
I'm not sure what you have access to there, but if you wanted to try meat instead of dry food, you basically want to avoid beef, chicken and probably pork. You feed this for 8 weeks, then if the diarrhoea improves introduce one meat protein a week to try and work out what triggers the diarrhoea again. Feeding just one meat source is not a balanced diet long-term, but once you work out what triggers her diarrhoea you can looking into either doing a home cooked diet that is nutritionally balanced (there are websites that can help you do this), or work out a cat food you have access to that is low allergen.
Some information here about IBD: http://www.veterinarypartner.com/Content.plx?A=598
The other option is to add in a probiotic to rebalance the gut flora in Lady's intestines. You could try something like this: https://www.lovethatpet.com/store/paw-digesticare-probiotic-powder/
Some cats will also respond to adding a little fibre to their diet, so you could sprinkle 1tsp of Benefibre or psyllium husks onto her food.
Hope that helps, let me know how you go.