Red pee is perfectly normal and just something that rabbits produce sometimes. It can be from plants or it can happen spontaneously (for no known reason).
if there’s blood in pee you can find out by dripping hydrogen peroxide on the pee spot. If it foams, it’s blood. If it doesn’t foam, it’s not blood. Forum leader erseygirl has tested this with her bunnies, and it works.
Cranberry juice is often recommended for urinary issues. You can use diet juice because its only a few drops. Read on the package, if the sweetener is sucralose (aspartame) or sodium saccharine, a few drops won’t hurt. We have members who have used cranberry successfully.
It’s very important that your bunny drinks water so he rinses out his bladder properly. Give him two bowls, one with plain water and one with cranberry water. Some buns wont drink at all if their water tastes differently.
Is he a good hay eater? If you cut out all other food, you must make sure he’s eating his hay. Some buns stop eating instead of trying the hay, and a bunny mustn’t go with an empty stomach for more than 6 hours max. Also if the bun only eats hay it’s extra important it drinks lots.
I have a friend who’s bun got a bout of sludge, that resolved itself in a couple of days. She was told to cut back on pellets and give more vegs. This was because vegs have so much fluid. Lettuce is good for example, kale should be avoided due to too high calcium content. Cilantro is good.
It’s absolutely great that his poop and activity level etc are normal.