Which cat is peeing blood?

[FreedomAnniversary]

We have 4 cats, 2 sets of 2 who have been living together for the past 7ish months. No fights but there is hissing.

I just went to give my child a shower and there was blood in the bath. I know one of the cats wees in the bath but we don't know which one. It wasn't thick blood, just rust coloured and had little 'bits' in it.

Two of the cats are outdoor, the other two are indoor, we have 4 litter trays in the house and they are 5 and 11 (two of each).

They all seem fine, eating and drinking etc but obviously I'm worried now.

Is there any way for me to find out which is ill other than stalking them all?

What sexes are they?

Neutered tons often get bladder crystals which makes them pee blood. It's generally from eating food with fish in. Switching to a non-fish food would fix it.

Ours gets this.

The 11 year olds are girls, the 5 year olds are a boy and a girl.

The younger 2 are the outdoor ones, one of the older ones will only go outside with me and the other refuses to go out under any circumstances.

We are currently feeding them a fish based food, will stop that in case it is boy cat but may take the older two to the vet for a general check up tomorrow anyway.

All of them are spayed/neutered as well.

@VikingLady

Neutered tons often get bladder crystals which makes them pee blood. It's generally from eating food with fish in. Switching to a non-fish food would fix it.

Ours gets this.

Where did you read that? Our vets (many, many of them over the years) have always said that cheap, dry food such as Go Cat is generally the culprit (something to do with magnesium).

OP. It could be any of them. I would try to split them up for a night, in different rooms, each with their own litter tray, and see what happens.

I have found the most likely cat and she is likely to have cystitis but will be fine! Just have to stalk her to get a urine sample.