Are Isopods Aggressive?

The issue here is defining aggressive. Will they attack you, no, will healthy isopods attack a reptile or other inhabitant, no. Will they attack each other? For most species no, but there are some species that will defend a territory.

Attacking other tank mates

Have a look online and you will find the odd mention of some species nipping some reptiles. The main species mentioned is Porcellio Laevis, the Dairy Cow Isopod. These are well known to require a lot of protein, in part because they breed quickly. This means that you need to offer them some form of protein. That can be in the form of fish food, although I would look at an insect-based food over a fish meal-based food. If they have their needs met then they are unlikely to have a nibble of any other enclosure inhabitants. But it's still worth having a look at an alternative if you have a sensitive reptile, or are planning on having very young animals in the enclosure with them. Dwarf White Isopods (Trichorhina Tomentosa) are considered a great choice for enclosures with small and sensitive animals.

Being territorial

Many species of Porcellio can be territorial, but given enough space you can still have a thriving colony. Some such as Dairy Cows will breed rapidly, which is both a huge advantage and a tiny issue. With a large number of individuals in one space, there is scope for cannibalisation.

Make sure their needs are met

 If your colony isn't growing, or you're seeing issues between individuals, then make sure their diet and environmental conditions are suitable. If they're all having to fight for a small space that is the only space that is humid enough for them then you'll have more issues than if they can utilise the entire vivarium. Make sure they're getting all the macro and micro nutrition they need. If there are too many then consider spiting the colony, and either setting up another enclosure or swapping with another keeper.

Understanding Isopod Behaviour

Isopods are scavengers, not predators, so there is no chance of them going after a reptile or similar. But some might not see a difference between a shed skin on the vivarium floor, and a shedding skin still on the reptile. This obviously can be annoying and stressful for the reptile.

 Some species, and here I’m mostly talking about the genus Porcellio, although it always pays to do your research before adding any isopod to your collection, have higher protein needs and are aggressive to others of the same species. Keepers have reported Porcellio species having a nibble of dead skin on their hands if they’ve not been fed the correct diet.

 The aggression levels likely come down to the evolution of their behaviour. Porcellio are a temperate species, meaning there will be times of the year that they are slower, they can not roll into a ball in the same way as Armadillium spp. Leaving aggression as their only defence.

 Now this isn't against a large predator, instead in that situation they stay still and hope they won't be noticed, but against similar-sized animals. Meaning that they will fight other Porcellio for territories and other important resources.

Dominating through numbers

 Some of the extremely fast breeding species might just dominate through sheer numbers, but it’s easy enough to keep on top of the numbers yourself and remove excess numbers.

How to Prevent Aggression in Isopod Colonies

 Feed an appropriate diet for the species you are keeping, both in terms of what you are feeding and how much.

Avoid overcrowding, and make sure they can use all the enclosure you are offering. If one half is too dry then you might start to see overcrowding at the other end.

Choose the right species for the set-up you are keeping them for. Don't add a species with high protein needs to a tank with sensitive animals.

If you have a sick reptile then consider moving them away from the isopods, ideally in a hospital tank. A reptile staying still for a long period of time might be disturbed by isopods moving around without considering any potential aggression.