Can you describe their diets? Soft mushy poops usually means too many rich foods in the diet (pellets, treats, starchy or sugary veggies).

Also, if they are unneutered, they will mark, and especially as you have two? How old are they?

Like Dana said, poop changes could be due to diet. Did you feed them something different recently?

Bunnies don’t recognise family units so while they may get along now, they may not in the future when they are older. It doesn’t matter whether they are from the same litter or not. How old are they? It’s best to get them neutered at the right age and then go through a proper bonding process to ensure the wellbeing of both.

For litter box habits, I find that it really depends on the bunny. Some are not neutered but have good litter box habits, some are just horrible at it, and some are in between. I don’t think 5ft and 4ft are too big of a space, but how long have you had them? Some people train their bunnies by giving them a smaller space initially because it’s easier to pinpoint where their favourite corner for pooping and peeing is, and then placing the litter box there. Once they are trained, then you can expand the area.

There’s no such thing as too many poops. More hay eating = more poops. It’s concerning that they’re mushy/soft though. What are their diets like?

Also, 5ft x 4ft isn’t really that big so I can’t imagine that’s why they’re not pooping 100% in their trays. It’s most likely because they’re young and not neutered.

If your rabbits are 10 or 12 weeks old or more, it’s really not safe to keep them in the same cage. It doesn’t matter if they are brothers or have grown up together, once their hormones come into play, there is a very good chance that they will start fighting. When rabbits fight, they can inflict severe injuries on each other. They can also come to dislike each other so much that they may not be able to bond in the future, even after they are neutered. They extra poop now could be a result of hormones and territorial feelings. You should separate them now, and not even let them out together at all for play and exercise – they will have to take turns. After they are neutered and have gone through the bonding process, then they can live together again.

“Mostly pellets and carrots”

You really need to get them eating hay. It needs to be the bulk of their diet. Do they have any hay at all? And carrots should be fed in moderation, i.e. as a treat.

Since they are adults now, you should definitely scale back the pellets, and eliminate or severely reduce the carrots (1/4″ piece a couple times a week would be OK). My 9 lb bun gets about 1/4″ cup of pellets per day, split into two meals, smaller bunnies should get less than that as adults. Carrots and pellets can definitely cause poopy butt. Unless they have a medical issue (or sometimes if the batch of hay is bad), they will eat their hay.

Once you get them used to eating their hay, you should start introducing leafy greens, as they are another important part of the diet. See this page for more info on diet and introducing greens safely:
https://binkybunny.com/BUNNYINFO/tabid/53/CategoryID/2/PID/940/Default.aspx

It can be tricky getting a stubborn bun to eat hay and I can totally understand your reluctance to reduce his pellets for fear of him starving. It’s going to be tough because you want him to he hungry enough (I. E. From not getting as many pellets) but if he genuinely doesn’t like the hay, there is the risk that he will go without. I don’t know if instinct will take over.

What you could do is get a few different hay types (see if you can buy samples to save ££ if he ends up not liking them) and try hand feeding a few strands to see his initial reaction. Many rabbits love orchard grass as it’s quite sweet. You could even try alfalfa which is usually for young buns or pregnant/nursing buns as it’s quite high in protein but it’s supposedly irresistible. It might help get him used to hay but then of course you’ll need to wean him onto a grass hay such as orchard, timothy, oat etc so keep that in mind, perhaps as a back up.

You could try feeding a very small amount of pellets in the morning (a tablespoon), do the hay taste test and if there’s any that he seems to enjoy, plonk that everywhere. Litter box, ball, pile on the floor etc. Then give him another small portion at night. Try to keep roughly 12 hours between non hay feedings. I do pellets at 5:30am, and fibafirst sticks at 5:30pm. Just hay in between.

Obviously as you’ll need to monitor his eating to see if he is munching on the hay it would be best to do this on a day that you’re home all day.

If he doesn’t appear to be eating, you could give him a tiny amount of pellets midday just so he has something in his tummy but it’ll really be a battle between who will give in first lol.

Carrots are like ice cream to bunnies. It isn’t really even about weight, it is about the function of their gut. Rabbits are basically tiny horses. Horses love apples and carrots, but they cannot eat ONLY apples and carrots as they would get very sick.

Rabbit and horse guts need lots and lots of fiber to function properly. They have microbes in their gut that allow them to digest hay and other vegetables where we could not.

You can ask you mom if she would allow you to eat ice cream and cookies for every meal. Even if she did, you would eventually feel very sick and would not get the nutrition you needed.

You could also point out that wild rabbits live on mainly grass, so you’re providing the natural diet for them, whereas carrots are bunny junk food.

You could also show her this article from the House Rabbit Society http://rabbit.org/the-importance-of-hay/

I read yesterday that rabbit’s front teeth grow 2mm a week, and their back teeth grow 3mm, which is why it’s SO important that they have hay to wear them down – pellets and veggies don’t wear them down, as hay and grass is covered in tiny spikes that work kind of like sandpaper on their teeth to keep them the correct length.

Lastly, you can appeal to her on the money aspect – bunnies with an incorrect diet are far more likely to need expensive vet care!