Hi Haelynx,
Welcome to PF

For your information, food like Smartheart, Friskies, Whiskas, Pro Diet (food off the shelf from supermarket) is usually not good for your cat as it has very low nutritional value. Also the salt content is very high and should your cat drinks very little water there is a high chance that your cat might get kidney stones in the later stage of their life. I would suggest to you to give your cats premium food like RC, Iams, or Blackwood depening on your budget

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If you choose Blackwood, I foresee no problems as most cats that eat BW are fine. Their poo is nicely formed and less odour compared to other brands. Before you change to BW, have you de wormed your cat? Worms in the stomach could be the cause of loose stools as well.